ath5k: split descriptor handling and frame receive

Move frame reception into it's own function to have a clearer separation
between buffer and descriptor handling and things that are done when we
actually receive a frame.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>