Input: elants_i2c - zero-extend hardware ID in firmware name

Let's zero-extend hardware id number when forming firmware file name,
to avoid kernel requesting firmware like "elants_i2c_   0.bin", which
is quite unexpected.

Acked-by: Charlie <Mooney&lt;charliemooney@chromium.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>