greybus: loopback: Add asynchronous backoff

A specific request from the firmware people is the ability to back-off from
sending more asynchronous operations once a specific number of operations
are in-flight.

This patch adds that ability - with a new sysfs parameter
'outstanding_operations_max' which controls the maximum number of
operations that can be outstanding/in-flight at any time.

When outstanding_operations_max contains a non-zero value and asynchronous
operations are being used - we will back-off until the completion counter
is < outstanding_operations_max. Tested in both synchronous and
asynchronous mode and with gb_loopback_connection_exit() interrupting
in-flight operations.

Suggested-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>