media: staging: rkisp1: params: in the isr, return if buffer list is empty

Currently the code in the isr checks if the buffer list is not
empty before referencing a buffer and then check again if the
buffer is not NULL. Instead we can save one 'if' statement by
returning if the buffers list is empty.
Also remove non-helpful inline doc 'get one empty buffer'

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>