net: remove duplicate code.

netdev_rx_handler_register() checks to see if the handler is already
busy which was recently separated into netdev_is_rx_handler_busy(). So
use the same function inside register() to avoid code duplication.
Essentially this change should be a no-op

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>