net: cfg802154: fix kernel-doc notation warnings

Add an enum heading to the kernel-doc comments to prevent
kernel-doc warnings.

cfg802154.h:174: warning: Cannot understand  * @WPAN_PHY_FLAG_TRANSMIT_POWER: Indicates that transceiver will support
 on line 174 - I thought it was a doc line

cfg802154.h:192: warning: Enum value 'WPAN_PHY_FLAG_TXPOWER' not described in enum 'wpan_phy_flags'
cfg802154.h:192: warning: Excess enum value 'WPAN_PHY_FLAG_TRANSMIT_POWER' description in 'wpan_phy_flags'

Fixes: edea8f7c

 ("cfg802154: introduce wpan phy flags")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Alexander Aring <alex.aring@gmail.com>
Cc: Stefan Schmidt <stefan@datenfreihafen.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230714045127.18752-3-rdunlap@infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>