canbus: isotp: send_sf() only checks length in ASSERT statement

[henrikbrixandersen]

Describe the bug
The send_sf() function in subsys/canbus/isotp/isotp.c only checks the SF length against the ISO-TP DL - index in an assert.

Expected behavior
It should always be checked at runtime.

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK
• Commit SHA: 00e9461

Additional context
Found by code inspection.