ath9k: simplify the code-paths when not using the built-in EEPROM

There were two paths in the code for "external" eeprom sources. The code
in eeprom.c only handled the cases where the eeprom data was loaded via
request_firmware. ahb.c and pci.c on the other hand had some duplicate
code which was only used when the eeprom data was passed via
ath9k_platform_data.
With this change all eeprom data handling is now unified in eeprom.c.

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