net: ieee802154: Use the IEEE802154_MAX_PAGE define when relevant

This define already exist but is hardcoded in nl-phy.c. Use the
definition when relevant.

While at it, also convert the type from uint32_t to u32.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20220125122540.855604-3-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>