carl9170 firmware: add flag for asynchronous commands

This flag can be used by the application to tell _same_
application that it does not have to wait for the command
response, or it does not need to parse the received
response if the flag is set.

NOTE: The firmware does not care about this flag and
processes any received command in the same way.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>