nl802154: fix cca mode wpan phy flag

This patch fix the handling to call cca mode setting. If the phy isn't
flag then the driver doesn't support this setting.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reported-by: Varka Bhadram <varkabhadram@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>