ixgbe/base: configure MDIO clock for X550em

ixgbe/base: configure MDIO clock for X550em

The MDIO clock needs to be configured for a specific speed for
x550em. We expected this to be done automatically, but in
the early days of the project this was not happening. We put
code in to do this ourselves.
Eventually, we decided that there is no harm in having SW do
this all the time, so we may not remove this code in the future.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>