netifd: interface: do not release device on link down

On IFPEV_DOWN, do not release the main device if the interface is intended to be brought back up again by interface_handle_config_change and there is no configuration change. This avoids releasing the device and then claiming it again when link is lost.

Releasing and then claiming the device can, if the driver is configured to power the PHY down when IFF_UP is cleared, cause the PHY to be power cycled. This can in combination with glitchy hardware and/or drivers where it takes a few seconds for the link to stabilize after the PHY has been powered up cause a loop where loss of link causes netifd to power cycle the PHY and when the PHY comes back up the link glitches and the cycle repeats.