powerpc/perf: Change type of the bhrb_users variable

This patch just changes data type of bhrb_users variable from
int to unsigned int because it never contains a negative value.

Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>