ASoC: da7219: Improve error checking of mclk enable/disable

Should only try to enable/disable the provided mclk, during bias
level changes, if it's not NULL. Also return value of
clk_prepare_enable() should be checked and dealt with accordingly.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Mark Brown <broonie@kernel.org>