media: atomisp: Remove atomisp_sensor_start_stream()

Now that we no longer have continuous mode we always want to
start the sensor on the first atomisp_start_streaming() call
and stop it on the first atomisp_stop_streaming() call.

Remove atomisp_sensor_start_stream() which returned the number
of streams which should be active before starting the sensor
and always start/stop the sensor directly.

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>