xen/events: fix error code in xen_bind_pirq_msi_to_irq()

Return -ENOMEM if xen_irq_init() fails.  currently the code returns an
uninitialized variable or zero.

Fixes: 5dd9ad32

 ("xen/events: drop xen_allocate_irqs_dynamic()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Juergen Gross <jgross@ssue.com>
Link: https://lore.kernel.org/r/3b9ab040-a92e-4e35-b687-3a95890a9ace@moroto.mountain


Signed-off-by: Juergen Gross <jgross@suse.com>