ASoC: da7219: Make more efficient use of MCLK within driver

Currently, if the driver has control of MCLK then it remains
enabled as long as the codec is in STANDBY or above. The MCLK is
only really required in STANDBY when a 3-pole jack is inserted
and the HP detect procedure is required to run.

This patch updates the code to enable/disable the MCLK when moving
between the STANDBY and PREPARE bias level, and when a 3-pole jack
is inserted and HP detection is required, thus saving power at all
other times.

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