ASoC: sgtl5000: Allow 8kHz playback in codec slave mode

When trying to play a 8kHz file with codec in slave mode we get the following
error on a mx28evk:

$ aplay -Dhw:0,0 stereo_8k.wav
Playing WAVE 'stereo_8k.wav' : Signed 16 bit Little Endian, Rate 8000 Hz, Stereo
[   21.218647] sgtl5000 0-000a: PLL not supported in slave mode
[   21.224559] sgtl5000 0-000a: 128 ratio is not supported. SYS_MCLK needs to be 256, 384 or 512 * fs
[   21.233687] sgtl5000 0-000a: ASoC: can't set sgtl5000 hw params: -22
aplay: set_params:1123: Unable to install hw params:

This error happens because we are using 'sys_fs' instead of 'frame_rate' in the
valid ratio check.

Use the real'frame_rate' so that the ratio is correctly calculated and the
playback can run.

sgtl5000 codec manual states that in 'Synchronous SYS_MCLK input' mode that the
following SYS_CLK frequencies are allowed: 256*fs, 384*fs, 512*fs.

, where fs is the sampling frequency, which can be in the range of:
8, 11.025, 16, 22.5, 32, 44.1, 48, 96 kHz.

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