ath9k_common: Move RX filter code to ath9k_htc

The calculation of RX filter is fairly different
between ath9k and ath9k_htc, trying to make this
common between the two drivers would result in code churn.

While at it, remove the handling of PSPOLL filter,
it can be added when(if) AP support is added to ath9k_htc.

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