igbvf: Remove unnecessary irq disable/enable

This irq disable/enable pair used to wrap access to the driver's vlgrp
struct, which is no longer present. So, then, this could also so no longer
be present.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>