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 <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/events/events_base.c

···
 		clear_evtchn_to_irq_row(row);
 	}
 
-	evtchn_to_irq[EVTCHN_ROW(evtchn)][EVTCHN_COL(evtchn)] = irq;
+	evtchn_to_irq[row][col] = irq;
 	return 0;
 }