media: atomisp: fixes build breakage for ISP2400 due to a cleanup

A temporary var needed for building with ISP2400 was removed
by accident on a cleanup patch.

Fix the breakage.

Fixes: 852a53a0 ("media: atomisp: get rid of unused vars")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>