[media] s5p-fimc: Prevent potential buffer overflow

Replace the hard coded csi_sensors[] array size with a relevant
constant to make sure we don't iterate beyond the actual array.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>