ACPI: SBS: Return rate in mW if capacity in mWh

klaptopd assumes rate to be in same units as capacity.

http://bugzilla.kernel.org/show_bug.cgi?id=9362

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>