ath9k_hw: fix antenna diversity on AR9285

On AR9285, the antenna switch configuration register uses more than just
16 bits. Because of an arbitrary mask applied to the EEPROM value that
stores this configuration, diversity was broken in some cases, leading
to a significant degradation in signal strength.
Fix this by changing the callback to return a 32 bit value and remove
the arbitrary mask.

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