ieee802154: rename and move WPAN_NUM_ defines

This patch moves the 802.15.4 constraints WPAN_NUM_ defines into
"net/ieee802154.h" which should contain all necessary 802.15.4 related
information. Also rename these defines to a common name which is
IEEE802154_MAX_CHANNEL and IEEE802154_MAX_PAGE.

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