media: atomisp: Add an index helper variable to atomisp_buf_done()

With the videobuf2 conversion accessing the index of a frame is going
to become more involved and writing this out each time is undesired.

Add an 'i' helper variable for the index and assing this once as
preparation for the videobuf2 conversion.

This also makes use of the new rules wrt max line-lengths in the kernel
to avoid breaking up a bunch of lines. Not breaking these lines results
in better readable code (IMHO).

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