ACPI / battery: minor tweaks to acpi_battery_units()

Make the acpi_battery_units() function take a const argument and return
a const char*, too. Also make it static. It probably doesn't matter, as
gcc will be clever enough to optimize and inline the code even without
these hints. However, we also get rid of a #ifdef block by moving the
function closer to its usage location, so it's at least a small gain in
code readability.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>