diag: Add subid mask ptr correctly to ms_ptr list

There is a possibility of corrupting the ms_ptr list
while adding new subid mask ptr to the list. The patch
ensures the addition of new ptr without corrupting list.

Change-Id: I79e16e86d46e74b4b5d9b7af1ec1b56a0db7f15d
Signed-off-by: Hardik Arya <harya@codeaurora.org>