clk: wm831x: Fix clk_register() error code checking

clk_register() returns an ERR_PTR upon failure, not NULL. Fix
these error paths.

Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>