intel_sst: MRST can only do mono recording

Fix bug

	$ arecord -Dplughw -c2
	Recording WAVE 'stdin' : Unsigned 8 bit, Rate 8000 Hz, Stereo
	arecord: set_params:1116: Unable to install hw params:
	ACCESS:  RW_INTERLEAVED
	FORMAT:  U8
	SUBFORMAT:  STD
	SAMPLE_BITS: 8
	FRAME_BITS: 16
	CHANNELS: 2
	[...]

Root cause is, the driver is reporting 2-channel capture capability
that is not supported by the MRST hardware. So the plughw plugin
end up requesting 2-channel capture which fails.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>