sh: clkfwk: Fix up rate rounding error handling.

According to the linux/clk.h definition we should be handing back an
errno value or a valid rate. This fixes up the case where 0 can be
returned for invalid frequencies or cases where rounding has no
selectable candidate.

Reported-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>