p54: fix WARN_ON at line 2247 of net/mac80211/rx.c

This patch hopefully fixes a mac80211<->p54 interaction problem, which was
described by Larry Finger (ref: http://marc.info/?l=linux-wireless&m=123009889327707 )

I guess the warning was triggered by pending frames in the receive queue,
while we're doing a band change 5GHz.

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