netifd:Make sure interface device config is also cached

netifd:Make sure interface device config is also cached

Fixes issue interface device config is not applied in some cases.
As the interface device config was applied but not always cached;
an interface device config diff was not always detected.

Simplify device config setting by exposing as api only device_apply_config

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>