can: raw: raw_getsockopt(): reduce scope of err

Reduce the scope of the variable "err" to the individual cases. This
is to avoid the mistake of setting "err" in the mistaken belief that
it will be evaluated later.

Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/all/20240220-raw-setsockopt-v1-1-7d34cb1377fc@pengutronix.de


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