mISDN: l1oip_core: replace _manual_ swap with swap macro

Make use of the swap macro and remove unnecessary variables skb and cnt.
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>