ACPI: battery: Fix missing NUL-termination with large strings

When encountering a string bigger than the destination buffer (32 bytes),
the string is not properly NUL-terminated, causing buffer overreads later.

This for example happens on the Inspiron 3505, where the battery
model name is larger than 32 bytes, which leads to sysfs showing
the model name together with the serial number string (which is
NUL-terminated and thus prevents worse).

Fix this by using strscpy() which ensures that the result is
always NUL-terminated.

Fixes: 106449e8

 ("ACPI: Battery: Allow extract string from integer")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>