thermal: power_allocator: req_range multiplication should be a 64 bit type

req_range is declared as a u64 to cope with overflows in the
multiplication of two u32.  As both req_power and power_range are u32,
we need to make sure the multiplication is done with u64 types.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>