Input: elan_i2c - don't require known iap version

The Asus X456UA has an ELAN1000 touchpad with IAP version 0xe.  This is
unknown to elan_get_fwinfo() so driver probe fails and I am left with an
unusable touchpad.

However, the fwinfo is not required for general driver usage, it is only
needed if the user decides to upload new firmware.

Adjust the driver so that we do not abort probe when we encounter
unexpected IAP version, but rather warn user that firmware update feature
of the driver will not work.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>