RDMA/nes: Change MDIO bus clock to 2.5MHz

Change the PHY clock divisor to make the MDIO clock 2.5MHz, instead of
3.5MHz (which is out of spec).

Signed-off-by: Tatyana Nikolova <Tatyana.E.Nikolova@intel.com>
Signed-off-by: Faisal Latif <Faisal.Latif@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>