ieee802154: fix gcc-4.9 warnings

All older compiler versions up to gcc-4.9 produce these
harmless warnings:

drivers/net/ieee802154/ca8210.c: In function 'ca8210_skb_tx':
drivers/net/ieee802154/ca8210.c:1947:9: warning: missing braces around initializer [-Wmissing-braces]

This changes the syntax to something that works on all versions
without warnings.

Fixes: ded845a7

 ("ieee802154: Add CA8210 IEEE 802.15.4 device driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>