clk: wm831x: Use devm_clk_register() to simplify code

Move this driver to use devm_clk_register() to simplify some
error paths and reduce lines of code.

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