greybus: connection: add connection-state locking

Add locking, and the implied barriers, to connection-state updates.

This will be used to fix a number of races in the operations and
connection-tear-down implementations.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>