nl802154: export supported commands

This patch will export the supported commands by the devices
to the userspace. This will be useful to check if HardMAC
drivers can support a specific command or not.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>