ath9k: fix compile warning on ath9k_hw_AR9287_check_eeprom()

CC [M]  drivers/net/wireless/ath/ath9k/eeprom.o
drivers/net/wireless/ath/ath9k/eeprom.c: In function ‘ath9k_hw_AR9287_check_eeprom’:
drivers/net/wireless/ath/ath9k/eeprom.c:2866: warning: comparison of distinct pointer types lacks a cast

Cc: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>