partitions: Mount storage for MTP after wipe

Conditionally run Add_MTP_Storage() after wipe if:
1) is wiped successfully
2) is storage (already implemented)
3) mount succeeds

Previous to this commit, Add_MTP_Storage() was called before mounting
storage, which fails. Mounting storage right after wipe should not be
a problem since Update_System_Details() does it anyways (and doesn't
complain if already mounted).

Change-Id: I8a5a78d5ac9181f3a9464dfe6b77284458f9868b