p54: fix oops on faulty devices

This patch fixes an oops when the devices suddenly starts
to receive martian data frames.

bug reference:
http://marc.info/?l=linux-wireless&m=122872280317635&w=2



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