ieee802154: atusb: store firmware version after retrieval for later use

The firmware versions will be used to enable selective features based on the
available firmware on the device. Make sure we do not need to fetch it for
every check but store it after the initial retrieval.

Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>