Input: elan_i2c - add product IDs FW names

Previously the elan_i2c touchpad driver would simply request the firmware
"/lib/firmware/elan_i2c.bin", which does not work well if there are
multiple such devices in the system.

Let's append the "product ID" (by using the same function as the sysfs
interface for consistency) to the filename.  This results in filenames of
the form "/lib/firmware/elan_i2c_72.0.bin", allowing you to support
multiple elan_i2c touchpads on the same device by simply naming each
device's FW with its corresponding product ID.  This way when you trigger a
fw update the driver will load the correct binary.

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