ath9k: fix AR9160 xpaBiasLvlFreq endianness handling

The xpaBiasLvlFreq parameter array is made up of 16 bit words which
aren't byte-swapped like the other 16-bit eeprom parameters are.
It's only used by the AR9160.

Signed-off-by: Adrian Chadd <adrian@freebsd.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>