mac802154: move ieee802154_dev.c to main.c

The ieee802154_dev functionality contains various function for
allocation and registration of an ieee802154_dev. This is equal to the
net/mac80211/main.c file. This patch rename the ieee802154_dev.c to
main.c to have the same behaviour.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>