ath9k: Fix FIF_PROMISC_IN_BSS processing in station mode

We must not disable ACK sending in this case since it would break
normal station operations. In addition, clarify the comment about AP
mode to make more sense.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>