p54: better firmware support

This patch hopefully contains all necessary changes to support
firmwares for all devices up to atleast 2.13.3.0.
(or: LowerMAC Protocol Rev: 5.5 )

And this is a big win, since:
 * newer firmwares are more stable and reliable than the old ones.
 * no problems anymore with packages > 1399 octets (without lowering the MTU).
 * monitor mode finally works on USB for more than just a few seconds.

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