can: slcan: slc_xmit(): use can_dropped_invalid_skb() instead of manual check

slcan does a manual check in slc_xmit() to verify if the skb is valid.
This check is incomplete, use instead can_dropped_invalid_skb().

Link: https://lore.kernel.org/all/20220514141650.1109542-2-mailhol.vincent@wanadoo.fr


Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>