Input: synaptics - Lenovo Thinkpad X1 Carbon G5 (2017) with Elantech trackpoints should use RMI

Lenovo use two different trackpoints in the fifth generation Thinkpad X1
Carbon. Both are accessible over SMBUS/RMI but the pnpIDs are missing.
This patch is for the Elantech trackpoint specifically which also
reports SMB version 3 so rmi_smbus needs to be updated in order to
handle it.

For the record, I was not the first one to come up with this patch as it
has been floating around the internet for a while now. However, I have
spent significant time with testing and my efforts to find the original
author of the patch have been unsuccessful.
Signed-off-by: Edvard Holst <edvard.holst@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/synaptics.c

@@ -173,6 +173,7 @@ static const char * const smbus_pnp_ids[] = {
 	"LEN0046", /* X250 */
 	"LEN004a", /* W541 */
 	"LEN0072", /* X1 Carbon Gen 5 (2017) - Elan/ALPS trackpoint */
+	"LEN0073", /* X1 Carbon G5 (Elantech) */
 	"LEN200f", /* T450s */
 	NULL
 };