ixgbe/base: set MDIO speed after MAC reset

ixgbe/base: set MDIO speed after MAC reset

The MDIO clock speed must be reconfigured after the MAC reset.
The MDIO clock speed becomes invalid, therefore the driver reads
invalid PHY register values. The driver now set the MDIO clock
speed prior to initializing PHY ops and again after the MAC reset.

As now the MDIO speed gets set in more than one place, make a
function for it so it will always be done correctly.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>