soundwire: mipi-disco: remove master_count property for masters

The master_count is only defined for a Controller or a Slave in the
MIPI DisCo for SoundWire document.

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