ath5k: no need to save/restore the default antenna

Since ath5k_hw_set_antenna_mode() always writes the default antenna register
and is called at the end of reset, there is no need to separately save and
restore the default antenna.

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