msm: kgsl: Remove sscanf() from kgsl_pwrctrl.c

Replace sscanf() in kgsl_pwrctrl.c with a generic function to safely
convert 32 bit unsigned ints.

CRs-fixed: 563585
Change-Id: Ic0dedbad10e2264ec8fccab622d094d123b49b77
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>