ath9k_htc: Fix RX filter calculation

Choose the MY_BEACON filter only in case of a single interface.
Also, set the ATH9K_RX_FILTER_MCAST_BCAST_ALL filter in case
of interfaces.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>