ath9k: Add an eeprom_ops callback for retrieving the eepmisc value

This allows deciding if we have to swap the EEPROM data (so it matches
the system's native endianness) even if no byte-swapping (swab16, based on
the first two bytes in the EEPROM) is needed.

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