ieee802154 socket: No need to check for ARPHRD_IEEE802154 in raw_bind().

ieee802154_get_dev() only returns devices that have dev->type ==
ARPHRD_IEEE802154, therefore, there is no need to check this again
in raw_bind().

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>