ath9k: Add support FIF_OTHER_BSS filtering mode.

Support for FIF_OTHER_BSS was missing.  This patch adds support for this
filtering mode which in turn resolves a problem where mesh interfaces would not
receive broadcast traffic.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>