p54: fix iwconfig txpower off

Disabling the receiver logic with P54_FILTER_TYPE_RX_DISABLED is not
supported by all firmwares. However we have an alternative: hibernation.
And the only side effect - so far - is a bit less power consumption. WIN!

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>