[media] s5p-fimc: Check return value of clk_enable/clk_set_rate

clk_set_rate(), clk_enable() functions can fail, so check the return
values to avoid surprises. While at it use ERR_PTR() value to indicate
an invalid clock.

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>