ASoC: Intel: mrfld - remove unnecessary check for pointer

the 'platform' pointer in sst_map_modules_to_pipe() is deref in caller
function so we need to check for it in this function

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>