can: proc: remove unnecessary variables

There is no need to define the variable "rate" to receive, just return
directly.

Link: https://lore.kernel.org/r/20210514100806.792-1-zuoqilin1@163.com


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