ixgbe: disable flow control by default

Since the adapter cannot tell what the remote end's flow control capability
is through auto-neg, we must turn off flow control by default.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>