Mms: Upon already initialized, return immediately

If the sInstance parameter is already initialized then there is
no need to create a new one, this will lead to memory leaks.
Just return immediately.

Change-Id: I68dc53766b5a7e37c591ad253fbd65cf6416658a
Author: Hongyu Zhang <hongyu.zhang@intel.com>
Signed-off-by: Hongyu Zhang <hongyu.zhang@intel.com>
Signed-off-by: Frederic Predon <frederic.predon@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 28037