media: atomisp: Drop ffmt local var from atomisp_set_fmt()

ffmt is a local variable pointing to a substruct of another local
variable which really just makes the code harder to read / follow,
so drop it.

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