ACPI: battery: Increase maximum string length

On the Dell Inspiron 3505, the battery model name
is represented as a hex string containing seven numbers,
causing it to be larger than the current maximum string
length (32).
Increase this length to 64 to avoid truncating the string
in such cases. Also introduce a common define for the length.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>