802.15.4g devices have a 2047 byte PDU.
So the assertion `netif->ipv6.mtu == IPV6_MIN_MTU` is too strict here.
This is only enforced on init, so changing the modulation at run-time
did not catch this bug.
To test, use e.g. `at86rf215` with
CFLAGS += -DAT86RF215_DEFAULT_PHY_MODE=IEEE802154_PHY_MR_OQPSK
fixes#14164