e1000e: cleanup use of check_mng_mode function pointer

Replace e1000_check_mng_mode() inline function with calls to the MAC ops
check_mng_mode function pointer.

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>