canbus: isotp: convert SF length check from ASSERT to runtime check

Convert the ISO-TP SF length check in send_sf() from __ASSERT() to a
runtime check.

Fixes: #61501

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
(cherry picked from commit 1b3d1e01deff942ef84cb08e9fa9b4c5e18de385)