samples: net: ieee802154: Update config to sane values

The IEEE 802.15.4 UART pipe config files in
echo_server/prj_qemu_802154.conf and echo_client/prj_qemu_802154.conf
did not had proper configuration options set. This prevents
qemu<->qemu communication between the echo-server and echo-client.

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