ixgbe: fix ring reallocation in ethtool

changing ring sizes in ethtool needs to be robust.  If an allocation fails the
driver must continue operation, with the previous settings.

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>