soundwire: intel_init: add checks on link numbers

Add mask to correctly read the SoundWire SHIM LCAP register. Only bits
2..0 are meaningful, the rest is about link synchronization and stream
channel mapping. Without this mask, the hardware information would
always be larger than whatever the BIOS would report.

Also trap the case with zero links.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>