mt76: mt7915: fix info leak in mt7915_mcu_set_pre_cal()

Zero out all the unused members of "req" so that we don't disclose
stack information.

Fixes: 495184ac ("mt76: mt7915: add support for applying pre-calibration data")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Felix Fietkau <nbd@nbd.name>