CC: ar71xx: fix ath79_soc_rev value for QCA9531 ver. 2

CC: 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.

Backport of r47262

Signed-off-by: Matteo Valentini <matteo.valentini@wiman.me>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@47787 3c298f89-4303-0410-b956-a3cf2f4a3e73