ar9170: use bitop macros for tx filter flags

This patch fixes a bug in configure_filter's (sub-)routines.
We never really cleared the flags once we updated the hardware state,
so we wasted our resources by applying already active settings.

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