[ Wi-Fi ] : set Tx power of wireless card

Ubuntu 12.04 test OK!

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example :

set Tx-Power in dBm :
iw dev wlan0 set txpower fixed 0
iw dev wlan0 set txpower fixed 30

restore to default:
iw dev wlan0 set txpower auto

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also to test this (work too):

iwconfig:

iwconfig wlan0 txpower 0

iwconfig wlan0 txpower 17