dummy-irq: require the user to specify an IRQ number

Make sure that we let the user know that without specifying IRQ#,
dummy-irq driver is useless.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>