media: atomisp: Drop streamoff_mutex

Now that __atomisp_streamoff() no longer drops isp->mutex to cancel
the watchdog timer, the streamoff_mutex is no longer necessary to
avoid multiple streamoffs racing with each other.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>