platform: msm_shared: Change pointer type to fix 0 GUID issue

It's "0" for mcfg partition while using char type pointer to get a
byte, so "total_part" is wrong , and the LAST LBA of userdata is not
updated.

Correct the typecasting while assigning last_partition_entry variable
and change the type for GUID checking.

Change-Id: I3324e1b747fc1b35415686cf0f024c0ad73d121d