device: fix bonding primary port selection

device: fix bonding primary port selection

The confuguration of the primary port was read
 but never used to properly configure the right port.

This patch fix this, and another small fix suggested
 by nbd to properly handle the "reload" of the
 primary interface when is changed.

Co-developed-by: Felix Fietkau <[email protected]>
Signed-off-by: Nicolò Veronese <[email protected]>
Link: https://github.com/openwrt/netifd/pull/49
Signed-off-by: Robert Marko <[email protected]>