ASoC: fsi: fixup channels_min/max

FSI can support 1 - 8 channel input/output,
but current driver is supporting 2 channel format only.
This patch fixes channel mismatch

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>