net: decnet: dn_nsp_in: use swap macro in dn_nsp_rx_packet

Make use of the swap macro and remove unnecessary variable tmp.
This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>