mac802154: check for really changes

This patch adds check if the value is really changed inside pib/mib.
If a transceiver do support only one value for e.g. max_be then this
will also handle that the driver layer doesn't need to care about
handling to set one value only.

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