ath9k_hw: clean up EEPROM endian handling on AR9003

Remove the double swapping of the descriptor data structure, instead
keep it little-endian (native format of the eeprom data), and byteswap
on access.
This allows sparse to verify endian access to the eeprom struct.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>