clk: sunxi: Fix checking return value of clk_register_[composite|factors]

clk_register_composite() and clk_register_factors() return ERR_PTR on error.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>