[media] s5p-fimc: Fix incorrect usage of IS_ERR_OR_NULL

Replace IS_ERR_OR_NULL with IS_ERR on clk_get results.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>