media: atomisp: Remove atomisp_streaming_count()

atomisp_streaming_count() is just an alias for isp->asd.streaming now,
replace it with directly checking that and remove the helper.

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