myri10ge: check that we can get an irq

Now that IRQ allocation is done in myri10ge_open(), we want to still
check when loading the driver that IRQ allocation could succeed later.

Additionaly, we fix the initialization and printing of netdev->irq.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>