ASoC: wm8994: Don't allow reconfiguration of FLL when it provides SYSCLK

Rather than trying to work around machine drivers which try to reprogram
the FLL while it is providing SYSCLK just return an error if they try.
This will avoid audio glitches during FLL reconfiguration, or at least
move the introduction of the glitches to the machine driver.

Since disabling the source for an active SYSCLK is not supported in the
first place systems shouldn't be doing this in the first place.

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