ieee802154: atusb: make two structures static, fixes warnings

The arrays atusb_chip_data and hulusb_chip_data are local to the source
and do not need to be in global scope, so make them static. Also
remove unnecessary forward declaration of atusb_chip_data.

Cleans up sparse warnings:
symbol 'atusb_chip_data' was not declared. Should it be static?
symbol 'hulusb_chip_data' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>