mt76: mt7615: add missing SPDX tag in mmio.c

Resolve the following checkpatch.pl script warning:

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>