net: rpl: Kconfig: Fix 'Network type' choice default

The 'Network type' choice always defaulted to NET_RPL_L2_ANY, because
choices prefer the first default with a satisfied condition (this was
true even when Zephyr still had the prefer-later-defaults patch).

Swap the defaults so that NET_RPL_L2_IEEE802154 becomes the default if
NET_L2_IEEE802154 is enabled, as intended.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>