xen: fix GCC warning and remove duplicate EVTCHN_ROW/EVTCHN_COL usage

This patch removes duplicate macro useage in events_base.c.

It also fixes gcc warning:
variable ‘col’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Joshua Abraham <j.abraham1776@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>