mt76: mt7915: fix return condition in mt7915_tm_reg_backup_restore()

Fix the issue that some registers not configured properly after
restarting testmode.
(e.g. change state from idle to off, and off to idle)

Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>