i40e: Never let speed get set to 0 in get_settings

In ethtool, there is a possibility of speed getting set to 0
if advertise is set to 0 (which it is when autoneg is disabled).
We never want this to happen as the firmware will actually attempt
to set the speed to 0 sending link down, so add an extra check
to make sure this doesn't happen.

Change-ID: I62e0eeee2cbf043d8e6f5c9c9f0b92794e877f01
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>