ath9k: Simplify and fix eeprom endianness swapping

The three eeprom implementations had quite some duplicate code when it
came to endianness swapping.
Additionally there was a bug in eeprom_4k and eeprom_9287 which
prevented the endianness swapping from working correctly, because the
swapping code was guarded within an "if (!ath9k_hw_use_flash(ah))". In
eeprom_def this check did not exist, so it seems that eeprom_def was the
only implementation where endianness swapping worked.

This patch takes the duplicate code and moves it from eeprom_* to
eeprom.c. The new code is derived from eeprom_def, while taking into
account the specifics from the other implementations.

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