canbus: isotp: fix single frame state machine stuck issue

it is too late to set ctx->state to ISOTP_TX_WAIT_FIN after send_sf
because send_state_machine could be called just between `send_sf` and
`ctx->state = ISOTP_TX_WAIT_FIN;`  in extremely case. like below:
```c
	ret = send_sf(ctx);
	-> send_state_machine (irq handler)
	ctx->state = ISOTP_TX_WAIT_FIN;
```
it will cause isotp_send never return.

Signed-off-by: Jiapeng Li <mail@jiapeng.me>