msm: kgsl: Change data type for ret in kgsl_mmap

ret variable is meant to store the return value which can be negative.
Change the data type from unsigned int to int.

Change-Id: I6ea46c40d22dfc6474bbfda9b0e7791d39798520
Signed-off-by: Kamal Agrawal <kamaagra@codeaurora.org>