greybus: timesync: Do not hold mutex on cancel_delayed_work_sync

There is a scenario where gb_timesync_svc_remove() can run, attain a mutex
and call cancel_delayed_work_sync(). In the meantime a worker may already
be running and trying to attain the same mutex but will never do so as the
gb_timesync_svc_remove() path is holding the mutex and waiting on the
delayed_work_sync() to complete - leading to deadlock. This patch addresses
by calling the cancel_delayed_work_sync() before locking the relevant
mutex.

Reported-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Tested-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>