media: atomisp: Remove source_pad parameter from functions and structs

Now that there is only 1 source-pad for an asd there is no need
to have a parameter for this in various places.

Remove the source_pad function parameter and
atomisp_sub_device.capture_pad data member.

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>