hal: Add the missing 'else' for mixer_paths filename selection

Fix the minor issue in function of query_platform. The 'else' is
missing, which causes that the mixer_paths file name of all the
8916 boards will be overwritten by the default one.

Change-Id: If105e38537f5d4d5d3f52dfc64ac62f11e829bbf
CRs-Fixed: 668629