ieee802154: cfg802154: make wpan_phy_class constant

Since commit 43a7206b ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the wpan_phy_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Message-ID: <20240305-class_cleanup-wpan-v1-1-376f751fd481@marliere.net>
[changed prefix from wifi to ieee802154 by stefan@datenfreihafen.org]
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>