ath9k: replace eeprom_param EEP_MINOR_REV with get_eeprom_rev

get_eeprom(ah, EEP_MINOR_REV) and get_eeprom_rev(ah) are both doing the
same thing: returning the EEPROM revision (12 lowest bits). Make the
code consistent by using get_eeprom_rev(ah) everywhere.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>