slip: sl_alloc(): remove unused parameter "dev_t line"

The first and only parameter of sl_alloc() is unused, so remove it.

Fixes: 5342b77c slip: ("Clean up create and destroy")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>