igb: fix i350 SR-IOV failture

When SR-IOV is enabled, i350 devices fail to pass traffic. This is due to
the driver attempting to enable RSS on the PF device, which is not
supported by the i350.

When max_vfs is specified on an i350 adapter, set the number of RSS queues
to 1.

This issue affects 2.6.39 as well.

CC: stable@kernel.org
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>