Fix issue when two MMS requests with same priority on both SUB's

When executing MMS request on one SUB and a new MMS request
is received on other SUB. First execute the current request
before handling new request.

CRs-Fixed: 958989
JIRA: CYNGNOS-2492

Change-Id: I57fee028d9ac0e91d94eb70f8ea75c7fcb5f7087