mt76: mt7663s: fix a possible ple quota underflow

Properly account current consumed ple quota in mt7663s_tx_pick_quota
routine and avoid possible underflow.

Fixes: 6ef2d665 ("mt76: mt7663s: split mt7663s_tx_update_sched in mt7663s_tx_{pick,update}_quota")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>