ath5k: apply the synth voltage tweak only on AR5112 rev 2

Might fix some stability issues on newer chips

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