arm_mpam: Stop using uninitialized variables in __ris_msmon_read()

Dan has reported two uses of uninitialized variables in __ris_msmon_read().
If an unknown monitor type is encountered then the local variable, now, is
used uninitialized. Fix this by returning early on error. If a non-mbwu
monitor is being read then the local variable, overflow, is not initialized
but still read. Initialize it to false as overflow is not relevant for csu
monitors.

Fixes: 823e7c3712c5 ("arm_mpam: Add mpam_msmon_read() to read monitor value")
Fixes: 9e5afb7c3283 ("arm_mpam: Use long MBWU counters if supported")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202512091519.RBwiJcSq-lkp@intel.com/
Closes: https://lore.kernel.org/r/202512100547.N7QPYgfb-lkp@intel.com/
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>