ath9k_hw: remove warning in ath9k_hw_def_get_num_ant_config

This patch fixes following warning

drivers/net/wireless/ath/ath9k/eeprom_def.c: In function 'ath9k_hw_def_get_num_ant_config'
drivers/net/wireless/ath/ath9k/eeprom_def.c:1425:47: warning: comparison
			between 'enum ath9k_hal_freq_band' and 'enum ieee80211_band'

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>