ath5k: reset more pointers after we free skbs

After we free skbs for receive or transmit descriptors, make sure we have no
pointers to the now invalid memory address.

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