media: atomisp: Remove unused lock member from struct atomisp_sub_device

The spin-lock embedded in struct atomisp_sub_device is not used anywhere,
remove it.

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>