ASoC: sgtl5000: Improve the error message on slave mode setting

For sgtl5000 to operate in slave mode it can only work in "Synchronous
SYS_MCLK input" mode.

In this mode only the following rates can be supported: 256*Fs, 384*Fs, 512*Fs.

Improve the error message to give a better indication as to why the clocking
failed for slave mode:

[   12.515399] sgtl5000 1-000a: PLL not supported in slave mode
[   12.524124] sgtl5000 1-000a: 233 ratio is not supported. SYS_MCLK needs to be 256, 384 or 512 * fs
[   12.535938] sgtl5000 1-000a: ASoC: can't set sgtl5000 hw params: -22

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