We only need to regenerate the sysfs files when the capacity units
change, avoid the update otherwise.

The origin of this issue is dates way back to 2.6.38:
da8aeb92
(ACPI / Battery: Update information on info notification and resume)

cc: <stable@vger.kernel.org>
Signed-off-by: NAndy Whitcroft <apw@canonical.com>
Tested-by: NRalf Jung <post@ralfj.de>
Signed-off-by: NLen Brown <len.brown@intel.com>