ath9k_htc: RX timestamp is reported at end of frame

Accurate RX timestamp reporting is important for proper IBSS merging,
mesh synchronization, and MCCA scheduling. Namely, knowing where the TSF
is recorded is needed to sync with the beacon timestamp field.

Tested with AR9271.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>