mac802154: remove channel attributes from sdata

These channel attributes was part of "channel context switch while xmit"
which was removed by commit dc67c6b3
("mac802154: tx: remove xmit channel context switch"). This patch
removes these unnecessary variables and use the current_page and
current_channel by wpan_phy struct now.

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