nl802154: stricter input checking for boolean inputs

So far we handled boolean input by forcing them with !! and assigning
them into a bool. This allowed userspace to send values > 1 which were
used as 1. We should be stricter here and return -EINVAL for all but
0 or 1.

Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>