ieee802154: remove default channel settings

This patch removes the default channel setting. A channel is always set
and there is no default channel setting according 802.15.4.

Drivers should set the default channel and page in probing routine. This
behaviour is currently a lack of all 802.15.4 drivers.

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