clk: sunxi: improve error reporting for the mux clock

clk_register_mux returns a pointer wrapped error value in case of
failure, so a simple NULL check is not sufficient to catch errors.
Fix that and elaborate on the failure reason on the way. The whole
function does not return any error value, so silently failing may
leave users scratching their heads because the kernel does not
provide any clues on what's wrong.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>