net/can: can_dropped_invalid_skb can be boolean

This patch makes can_dropped_invalid_skb return bool due to this
particular function only using either one or zero as its return
value.

No functional change.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>