avdtp: fix race condition when starting a stream

The AVDTP spec allows for a race condition between remote and local
device when issuing an AVDTP_START cmd on a stream in the OPEN state.

However, the internal state must continue to be consistent. For example,
suppose that avdtp_start() has been called while in the OPEN state and
a AVDTP_START cmd is sent.  Now before we have received a response (and
thus entered the STREAMING state), we *receive* a START cmd. Prior to
this fix, since the sep is still in the OPEN state, we would accept
the new START cmd. This will leads us to send both a Start_Ind and
Start_Cfm - not good.

Now, we track this transitional state (starting == TRUE).

NB - 'starting' is only in a valid state while the sep is in the
OPEN state. 'starting' is reset when we return to the OPEN state.
1 file changed