HID: wacom: remove unnecessary 'connected' variable from EKR

The 'connected' variable was poorly named, and this has led to some
confusion. We can get the same information by checking if a serial number
exists in the specified EKR slot.

Signed-off-by: Aaron Skomra <skomra@gmail.com>
Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>