ASoC: sgtl5000: Do a sanity check on SYS_MCLK

According to the sgtl5000 datasheet the valid range for SYS_MCLK is from 8 to
27 MHz.

Add a sanity check prior to enabling SYS_MCLK.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>