snd-firewire-lib: add amdtp_stream_callback() as a first callback

When first callback of context is executed, this module realize that the
stream starts. It's enough that The struct amdtp_stream.run should be changed
one time.

This new function is called one time from each context and do it, then overwrites
function pointer with appropriate callback functions.

This is a tricky to me because it was hard to understand old snd-firewire. So I
indicate it as apparently as possible.