ehci-dbgp: fix selecting n-th ehci controller

The ehci<n> number was parsed but ignored.

Fixes: 322ecbe4ac ("console: add EHCI debug port based serial console")
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/char/ehci-dbgp.c

@@ -1480,7 +1480,7 @@ void __init ehci_dbgp_init(void)
         unsigned int num = 0;
 
         if ( opt_dbgp[4] )
-            simple_strtoul(opt_dbgp + 4, &e, 10);
+            num = simple_strtoul(opt_dbgp + 4, &e, 10);
 
         dbgp->cap = find_dbgp(dbgp, num);
         if ( !dbgp->cap )