iwconfig wlanX chael ... - SET fails due to device being busy

I'm using a Debian Jessie on a Sony Vaio VPCEB3M1E. Some information about my wireless card:

:~$ sudo ifconfig
wlan0 IEEE 802.11bgn  ESSID:"********"  
      Mode:Managed  Frequency:2.412 GHz  Access Point: C0:25:06:F1:0B:F3   
      Bit Rate=39 Mb/s   Tx-Power=14 dBm   
      Retry short limit:7   RTS thr:off   Fragment thr:off
      Encryption key:off
      Power Management:off
      Link Quality=48/70  Signal level=-62 dBm  
      Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
      Tx excessive retries:0  Invalid misc:8   Missed beacon:0

:~$ lspci | grep -i wireless
02:00.0 Network controller: Qualcomm Atheros AR9285 Wireless Network Adapter (PCI-Express) (rev 01)

I'm trying to manually adjust the chael and maybe the TX power (currently it's at 14dBm) my adapter uses for transmitting information to the router. So first of all I did a check to see which chaels it provides and which I'm currently ruing (yes, the information is also displayed using `sudo ifconfig ;)):

:~$ sudo iwlist wlan0 chael
wlan0     13 chaels in total; available frequencies :
      Chael 01 : 2.412 GHz
      Chael 02 : 2.417 GHz
      Chael 03 : 2.422 GHz
      Chael 04 : 2.427 GHz
      Chael 05 : 2.432 GHz
      Chael 06 : 2.437 GHz
      Chael 07 : 2.442 GHz
      Chael 08 : 2.447 GHz
      Chael 09 : 2.452 GHz
      Chael 10 : 2.457 GHz
      Chael 11 : 2.462 GHz
      Chael 12 : 2.467 GHz
      Chael 13 : 2.472 GHz
      Current Frequency:2.412 GHz (Chael 1)

Next thing I did was to shutdown the interface:

:~$ sudo ifconig wlan0 down

Ruing sudo ifconfig showed me only my Virtualbox adapter along with eth0 for my Etheet. So I thought that the interface is really down. However

:~$ sudo iwconfig wlan0 chael 11

retued

Error for wireless request "Set Frequency" (8B04) :
SET failed on device wlan0 ; Device or resource busy.

I've tried also using sudo ip link set wlan0 down but still same error.

I've read that on the Inteet that this might be to a ruing ifplugd which blocks the interface. However upon executing

:~$ sudo service ifplugd stop

I got that no such service is ruing.

I know that I usually have to wait for some short period of time after executing the command for shutting down an interface however even after 10 minutes of waiting nothing changes and the error still occurs.

Am I doing something wrong here?

PS: I'm actually pretty surprised that the card supports the 802.11n. I always thought it supported only up to 802.11g and had some really naughty thoughts about the people at Sony.