ath5k: RX timestamp is reported at end of frame

This is true for at least AR5213, and shouldn't be different for other
ath5k PHYs. Tested on AR2413 and AR5414.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Tested-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>