ath5k: move checks and stats into new function

Create a new function ath5k_receive_frame_ok() which checks for errors, updates
error statistics and tells us if we want to further "receive" this frame or
not. This way we can avoid a goto and have a cleaner separation between buffer
handling and other things.

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