ath9k_htc: Fix TKIP encryption

Set IEEE80211_HW_RX_INCLUDES_FCS to indicate that
the FCS is present in RX frames. Also, remove a redundant
assignment of skb length and include the FCS_LEN
when checking padding.

Fixing this issue makes TKIP work.

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