[netdevice] Fix misleading comment on netdev_rx()

Unlike netdev_rx_err(), there is no valid circumstance under which
netdev_rx() may be called with a null I/O buffer, since a call to
netdev_rx() represents the successful reception of a packet.  Fix the
code comment to reflect this.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

@@ -478,7 +478,7 @@ static void netdev_tx_flush ( struct net_device *netdev ) {
  * Add packet to receive queue
  *
  * @v netdev		Network device
- * @v iobuf		I/O buffer, or NULL
+ * @v iobuf		I/O buffer
  *
  * The packet is added to the network device's RX queue.  This
  * function takes ownership of the I/O buffer.