can: raw: use guard clause to optimize nesting in raw_rcv()

We can use guard clause to optimize nesting codes like
if (condition) { ... } else { return; } in raw_rcv();

Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Link: https://lore.kernel.org/all/0170ad1f07dbe838965df4274fce950980fa9d1f.1661584485.git.william.xuanziyang@huawei.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>