ramips: use phytpt trigger for mt76 wireless

With this change, the LED trigger is independent from the (wireless)
netdev name. The (wireless) netdev name can be easiliy changed in
OpenWrt and would require an update of the netdev trigger settings each
time it is done.

This change is (for now) applied only to MT7628 devices from TP-Link, as
we only had the possibility to test this change against two of those
devices, namely a TL-WR841 v13 and a Archer C50 v3.

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