xen/pvcalls: remove redundant check for irq >= 0

This is a moot point, but irq is always less than zero at the label
out_error, so the check for irq >= 0 is redundant and can be removed.

Detected by CoverityScan, CID#1460371 ("Logically dead code")

Fixes: cb1c7d9b

 ("xen/pvcalls: implement connect command")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>