samples: net: echo_server: Decrease SRAM for atsamr21_xpro

In the Zephyr v3.5 to build the net samples pair echo-server/client the
SRAM requirements increased for the atsamr21_xpro board when passing
-DEXTRA_CONF_FILE=overlay-802154.conf argument. This adjusts example
features to free necessary SRAM to build and run the example.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>