mkiss: remove redundant check on len being zero

The check on len is redundant as it is always greater than 1,
so just remove it and make the printk less complex.

Detected by CoverityScan, CID#1226729 ("Logically dead code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>