networkd: link_enter_configured remove assert (#4800)

When we are in link_enter_configured we assume that the
link->state should be LINK_STATE_SETTING_ROUTES but in some
situation it's LINK_STATlE_SETTING_ADDRESSES.

Just ignore the wrong state.

Also since the return value not used any where
make link_enter_configured return type void.

Fixes: #4746