ASoC: sgtl5000: fix codec clock source transition to avoid clockless moment

Powering down PLL before switching to a mode that does not use it
is a bad idea. It would cause the SGTL5000 be without internal
clock supply, especially on the I2C interface, which would make
subsequent access to it fail.

Thus, in case of not using PLL any longer, first set the mode
control, then power down PLL.

Signed-off-by: Oskar Schirmer <oskar@scara.com>
Signed-off-by: Mark Brown <broonie@linaro.org>