ixgbe: Introduce function to control MDIO speed

Move code that controls MDIO speed into a new function because
there will be more MACs that need the control.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>