sh: Tidy up pre-clkdev clk_get() error handling.

clk_get() used to return NULL or an errno value depending on whether a
clkdev lookup failed or a clock wasn't found in the primary clock list.
As these disjoint paths were unified and everything now is handled via
clkdev lookups, the NULL case never makes it out of clk_get(). Update
accordingly and always look to the errno value.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>