ethdev: forbid MTU set before device configure

ethdev: forbid MTU set before device configure

rte_eth_dev_configure() always sets MTU to either dev_conf.rxmode.mtu
or RTE_ETHER_MTU if application doesn't provide the value.
So, there is no point to allow rte_eth_dev_set_mtu() before since
set value will be overwritten on configure anyway.

Fixes: 1bb4a528c41f ("ethdev: fix max Rx packet length")

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>