ath5k: honor FIF_BCN_PRBRESP_PROMISC in STA mode

We were setting RX_FILTER_BEACON even after entering STA mode,
which leads to a lot of unnecessary wakeups.  This should fix the
bug "Ath5k driver has too many interrupts per second at idle" at
http://bugzilla.kernel.org/show_bug.cgi?id=11749.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>