ieee802154: Introduce a helper to validate a channel

This helper for now only checks if the page member and channel member
are valid (in the specification range) and supported (by checking the
device capabilities). Soon two new parameters will be introduced and
having this helper will let us only modify its content rather than
modifying the logic everywhere else in the subsystem.

There is not functional change.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20230103165644.432209-4-miquel.raynal@bootlin.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>