net: dsa: mv88e6xxx: fix races between lock and irq freeing

free_irq() waits until all handlers for this IRQ have completed. As the
relevant handler (mv88e6xxx_g1_irq_thread_fn()) takes the chip's reg_lock
it might never return if the thread calling free_irq() holds this lock.

For the same reason kthread_cancel_delayed_work_sync() in the polling case
must not hold this lock.

Also first free the irq (or stop the worker respectively) such that
mv88e6xxx_g1_irq_thread_work() isn't called any more before the irq
mappings are dropped in mv88e6xxx_g1_irq_free_common() to prevent the
worker thread to call handle_nested_irq(0) which results in a NULL-pointer
exception.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>