hw/net/rtl8139: Simplify if/else statement

Rewrite:

      if (E) {
          return A;
      } else {
          return B;
      }
      /* EOF */
  }

as:

      if (E) {
          return A;
      }
      return B;
  }

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>