Fixes low 5 GHz tx power issues
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r46497

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46499 3c298f89-4303-0410-b956-a3cf2f4a3e73