fakelb: use own channel and page attributes

This patch adds an own phy attribute for page and channel into
fakelb_phy. The current way is to use the internal mac802154 stored phy
pib values which can occur in locking issues while using it inside the
driver layer.

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