e1000e: 82574 intermittently fails to initialize with manageability f/w

The driver can fail initializing the hardware when manageability firmware
is performing concurrent MDIO operations because the hardware semaphore
scheme to prevent concurrent operations between software and firmware is
incorrect for 82574/82583.  Instead of using the SWSM register, the driver
should be using the EXTCNF_CTRL register.  A software mutex is also added
to prevent simultaneous software threads from performing similar concurrent
accesses.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>