ath9k_hw: fix setting the hardware diversity flag

ath9k_hw_set_diversity is only called from init.c where it cannot affect
the hardware setting because it's cleared on the next reset.
Instead of using a PHY op for something that's supposed to be initialized
statically, set the register value directly in the INI override function.

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