media: atomisp: Stop direct calls to queue num_buffers field

Use vb2_get_num_buffers() to avoid using queue num_buffers field directly.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Hans de Goede <hdegoede@redhat.com>
CC: Sakari Ailus <sakari.ailus@linux.intel.com>