mt76: mt7921: fix crash when startup fails.

If the nic fails to start, it is possible that the
reset_work has already been scheduled.  Ensure the
work item is canceled so we do not have use-after-free
crash in case cleanup is called before the work item
is executed.

This fixes crash on my x86_64 apu2 when mt7921k radio
fails to work.  Radio still fails, but OS does not
crash.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>