jack: Update poll_fd also in draining state

jack: Update poll_fd also in draining state

... to support snd_pcm_drain for the JACK IO plugin.

With this changes there will be an poll_fd event in DRAINING state even if
the min_avail was not yet reached. Otherwise the application would never
recognize that all samples were processed by JACK.
In addition the JACK real-time thread is also processing when in DRAINING
state and not only when in RUNNING or PREPARE state.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>