Input: elan_i2c - expand maximum product_id form 0xFF to 0xFFFF

In order to support more projects in the future, we expand the
maximum product_id value form 0xFF to 0xFFFF.

Signed-off by: Duson Lin <dusonlin@emc.com.tw>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>