greybus: connection: add unidirectional enabled state

Add a new connection state ENABLED_TX in which only outgoing operations
are allowed.

This allows drivers to query the device during probe before allocating
their state containers without having to worry about racing incoming
requests.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>