net: l2: ieee802154: fix settings order

In the IEEE 802.15.4 area certain settings must be set before
net_if_up() may be called (e.g. the channel).

Also net_if_up() may not be called if
CONFIG_IEEE802154_NET_IF_NO_AUTO_START=y.

This fixes the set-up order and handling of
CONFIG_IEEE802154_NET_IF_NO_AUTO_START.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>