ASoC: da7218: Improve driver efficiency with regards to MCLK usage

Currently MCLK remains enabled during bias STANDBY state, and this
is not necessary. This patch updates the code to handle enabling
and disabling of MCLK, if provided, when moving between STANDBY
and PREPARE states, therefore saving power when no active streams
present.

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