mt76: mt7915: fix a signedness bug in mt7915_mcu_apply_tx_dpd()

"idx" needs to be signed for the error handling to work.

Fixes: 495184ac

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