Input: elan_i2c - set input device's vendor and product IDs

Previously the "vendor" and "product" IDs for the elan_i2c driver simply
reported 0000.  This patch modifies the elan_i2c driver to include the
Elan vendor ID and the touchpad's product id under
input/input*/{vendor,product}.

Specifically, this is to allow us to apply a generic Elan gestures config
that will apply to all Elan touchpads on ChromeOS.  These configs  match to
input devices in various ways, but one major way is by matching on vendor
ID.  Adding this patch allows the default Elan touchpad config to be
applied to Elan touchpads in this kernel by matching on devices that have
vendor ID 04f3.

Note that product ID is also available via custom sysfs entry "product_id"
as well.

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