interfaces/kiss: fix KISS interface MTU check on RX
interfaces/kiss: fix KISS interface MTU check on RX

`driver->rx_packet` holds each (temporally incomplete) incoming CSP packet,
while `driver->rx_length` has said buffer's valid number of bytes.

As `interface->mtu` only applies to CSP "data" (and `driver->rx_packet` is
filled from `driver->rx_packet->id.ext`, not `driver->rx_packet->data`), size
checks must take `CSP_HEADER_LENGTH` into account.

Whatsmore, when "End char" is processed, this is correctly being handled, as
evidenced by the following snippet:

```
/* The CSP packet length is without the header */
driver->rx_packet->length = driver->rx_length - CSP_HEADER_LENGTH;
```

satlab/libcsp,satlab/libcsp,GomSpace/libcsp,GomSpace/libcsp,satlab/libcsp,GomSpace/libcsp,libcsp/libcsp,libcsp/libcsp