mt76: mt7915: do not read rf value from efuse in flash mode

Do not read rf value from efuse when driver is configured to
use flash mode.

Tested-by: Bruce Chuang <bruce-ss.chuang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>