drivers: ieee802154: uart: Do not send FCS bytes to peer

No need to send FCS bytes as the monitor_15_4 is configured to not
expect them. If we change the monitor_15_4 to use them, then we would
need to put correct values into these two FCS bytes.
So cleanest solution is not to send FCS bytes at all.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>