ASoC: Support non-crystal master clocks for WM8731

Instead of unconditionally enabling the crystal oscillator on the WM8731
only enable it when explicitly selected via set_sysclk(), allowing machine
drivers to specify that they drive a clock into MCLK alone. This avoids
any conflicts between the oscillator and the external MCLK source and saves
power for systems which do not need the oscillator.

This should also deliver a small power saving on systems using the crystal
since the oscillator will only be enabled when the ADC or DAC is active.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>