ath9k_hw: fix duplicate (and partially wrong) definition of AR_CH0_THERM

AR_PHY_65NM_CH0_THERM and AR_CH0_THERM were supposed to refer to the
same register, however they had different SREV checks.

Remove the duplicate and use the checks. Since there were other SREV
checks present in the only place that uses this, this will probaby not
affect runtime behavior.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>