Change M4OSA_ERR_CREATE to return unsigned integer.

While M4OSA_ERR is defined as M4OSA_UInt32, MOSA_ERR_CREATE
is defined to return M4OSA_Int32. This leads to signed/unsigned
comparison warnings. M4OSA_ERR_CREATE has been changed to return
M4OSA_UInt32 to fix this issue.

Change-Id: I71a5c50a95c7f296469604b486a1d3969d302a3f
Signed-off-by: Ashok Bhat <ashok.bhat@arm.com>