ath9k_hw: fix antenna diversity on AR9285

ath9k_hw: fix antenna diversity on AR9285

commit 601e0cb165e65dc185b31fe7ebd2c0169ea47306 upstream.

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>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>