HID: multitouch: warn on sysfs group creation failure

This adds a warning message stating that the sysfs group was not able to be
created for the passed hid_device structure pointer with dev_warn.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Reviewed-by: Benjamin Tissoires <benajmin.tissoires@redhat.com>
[jkosina@suse.cz: massaged changelog a bit]
[jkosina@suse.cz: reformatted source]
Signed-off-by: Jiri Kosina <jkosina@suse.cz>