mt76: mt7663s: fix a possible ple quota underflow

mt76: mt7663s: fix a possible ple quota underflow

BugLink: https://bugs.launchpad.net/bugs/1910111
[ Upstream commit 1c79a190e94325e01811f653f770a34e816fdd8f ]

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

Fixes: 6ef2d665f64d ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>