clk-sunxi: fix a missing-check bug in sunxi_divs_clk_setup()

In sunxi_divs_clk_setup(), 'derived_name' is allocated by kstrndup().
It returns NULL when fails. 'derived_name' should be checked.

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>