soc: qcom: rpm_master_stat: Fix buffer overflow

Buffer overflow can happen when finding next set bit
due to type casting of uint32_t to unsigned long.

Fix this to correctly print number of active cores in
rpm_master_stat.

Change-Id: Ibeacc5ac66535e373965d8f8e4919829367cc257
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>