wifi: mt76: mt7925: cancel mlo_pm_work on stop

In the Linux kernel, the following vulnerability has been resolved:

wifi: mt76: mt7925: cancel mlo_pm_work on stop

mt7925 queues mlo_pm_work with a 5 second delay during multi-link
power-save setup and never cancels it on the stop path. If the device is
torn down inside that window, the work outlives the teardown and its timer
fires afterwards, trying to queue onto the workqueue that is already gone:

workqueue: cannot queue mt7925_mlo_pm_work [mt7925_common] on wq phy0
WARNING: kernel/workqueue.c:2283 at __queue_work+0x59/0xa0, CPU#1: swapper/1/0
call_timer_fn+0x2a/0x140
__run_timers+0x203/0x330
run_timer_softirq+0x86/0xf0

mt7921 already has its own stop callback, so add one for mt7925 that
cancels the work before calling mt792x_stop(). mt7925_ops backs both the
PCIe and USB drivers, so this covers both.