mt7601u: set promiscous mode based on FIF_OTHER_BSS

Most drivers use FIF_OTHER_BSS to set promiscous mode.  Let us
follow their lead even though it doesn't match exactly the HW
filter flags.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>