soundwire: stream: simplify sdw_alloc_master_rt()

soundwire: stream: simplify sdw_alloc_master_rt()

Only do the allocation in that function, and move check for allocation
in the caller. This will it easier to split allocation and
configuration.

No functionality change in this patch.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20220126011715.28204-10-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>