ath5k: fix off-by-one in gpio checks

Sanity checks against AR5K_NUM_GPIO were all broken.  This doesn't
currently cause any problems since we only use the first four gpios.

Changes-licensed-under: ISC

Reported-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>