media: staging: media/atomisp: don't compile unused code

currently the functions defined in ibuf_ctrl_rmgr.c file are only
used by isys_init.c when CONFIG_VIDEO_ATOMISP_ISP2401 is selected
so dont't compile it when not needed, also fixes some sparse warnings

Link: https://lore.kernel.org/linux-media/YDEZu3oniT/vUMMY@karthik-strix-linux.karthek.com


Signed-off-by: karthik alapati <mail@karthek.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>