interface: set interface in TEARDOWN state when checking link state

interface: set interface in TEARDOWN state when checking link state

When launching PROTO_CMD_TEARDOWN in interface_check_state() the interface
was set in IFS_DOWN state. In case an interface is now brought into IFS_SETUP
state in __interface_set_up() it will launch PROTO_CMD_SETUP trying to
bring the proto shell handler in S_SETUP state which will fail as the proto
shell handler is still in the S_TEARDOWN state.

Fix this by setting the interface in IFS_TEARDOWN state when the PROTO_CMD_TEARDOWN
event is launched which will prevent the interface being brought into IFS_SETUP state
as long as it's not in the IFS_DOWN state.

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