e1000e: cleanup e1000_init_mac_params_82571()

Combine two switch statements into one, convert a nebulous pointer to one
that is a bit more in keeping with the rest of the driver code and cleanup
some coding style.  No change in functionality, just cosmetic changes.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>