ieee802154: remove mlme get_phy callback

This patch removes the get_phy callback from mlme ops structure. Instead
we doing a dereference via ieee802154_ptr dev pointer. For backwards
compatibility we need to run get_device after dereference wpan_phy via
ieee802154_ptr.

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