e1000e: workaround invalid Tx/Rx tail descriptor register write

When the Manageability Engine (ME) is enabled on 82579, it periodically
accesses some MAC CSR registers.  There is an arbiter in hardware which
prevents simultaneous access of these registers by the host software, i.e.
the driver.  There is a hardware bug in the aribter that signals a host
access of the registers later than it actually happens.  A write of the
Transmit or Receive Descriptor Tail register could result in an incorrect
value if the driver and ME perform simultaneous accesses which could result
in an access to an invalid memory address.  This would return an
Unsupported Request which could hang the hardware.  Workaround the issue by
checking the FWSM register bit24 which is set by ME before it accesses the
MAC CSR registers.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>