openwrt: mask nested A-MSDU support for mesh

This disables A-MSDU support on mesh-links. This restores throughput on
mesh-links established between broken (ath10k) and correct (mt76)
implementations of mesh A-MSDU.

This patch can be dropped when switching to the next OpenWrt release, as
proper fixes have been applied.

Link: https://www.spinics.net/lists/linux-wireless/msg232410.html
Link: https://github.com/openwrt/mt76/issues/450



Signed-off-by: David Bauer <mail@david-bauer.net>