msm: msm_bus: Modify GC upperbound and fix the bimc_div function

The max value for grant count is the max of a signed 16 bit value
not unsigned 16 bit value which is what the driver had assumed.
Modified the bimc_div function to correctly handle cases where numerator
is smaller than denominator.

Change-Id: I0e104390a9c32f36c698c3840c8d55116e215edd
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>