mt76: remove variable 'val' set but not used

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/mediatek/mt76/mt76x0/phy.c: In function mt76x0_phy_rf_init:
drivers/net/wireless/mediatek/mt76/mt76x0/phy.c:1158:5: warning: variable val set but not used [-Wunused-but-set-variable]

Fixes: 10de7a8b ("mt76x0: phy files")
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>