p54: enable proper frame injection

This patch enables frame injection in monitor mode for all p54 devices.
As a result, any user can finally use the aircrack-ng suite out of the box.

e.g:
aireplay-ng --test wlan0
Trying broadcast probe requests...
Injection is working!
Found 1 AP

Trying directed probe requests...
XX:XX:XX:XX:XX:XX - channel: i - 'SSID'
Ping (min/avg/max): 1.536ms/3.193ms/4.377ms Power: 193.00
30/30: 100%

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