e1000e: only perform ESB2 MDIC workaround on certain configurations

A workaround added for all ESB2 devices (adds a delay for all MDIC accesses
which resolves an issue with the MDIC ready bit being set prematurely) is
applicable only to devices in which the MAC-PHY interconnect is not
operating in a certain mode with in-band MDIO.  Check the control register
for the operating mode and enable the workaround accordingly.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>