mt76: move set_{tx,rx}_path routines in mt76x02-lib module

Move mt76x02_phy_set_rxpath and mt76x02_phy_tx_dac routines in
mt76x02_phy.c since they are shared between mt76x2 and mt76x0 drivers.
Moreover move chainmask variable from mt76x2/mt76x0 to mt76_dev data
structure

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>