ar71xx: fix ath79_soc_rev value for QCA9531 ver. 2

ath9k expects to get revision id 2 for the QCA9531 ver. 2 rev. 0. This
fixes the very low TX power on some devices like the TP-LINK
TL-WR841ND v10.

As ath79_soc_rev is only used to get the revision number to ath9k on the
QCA9533, just set it to the expected value on the ver. 2.