p54: fix tx feedback status flag check

Michael reported that p54* never really entered power
save mode, even tough it was enabled.

It turned out that upon a power save mode change the
firmware will set a special flag onto the last outgoing
frame tx status (which in this case is almost always the
designated PSM nullfunc frame). This flag confused the
driver; It erroneously reported transmission failures
to the stack, which then generated the next nullfunc.
and so on...

Cc: <stable@kernel.org>
Reported-by: Michael Buesch <mb@bu3sch.de>
Tested-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>