mISDN: Add feature via MISDN_CTRL_FILL_EMPTY to fill fifo if empty

This prevents underrun of fifo when filled and in case of an underrun it
prevents subsequent underruns due to jitter.
Improve dsp, so buffers are kept filled with a certain delay, so moderate
jitter will not cause underrun all the time -> the audio quality is highly
improved. tones are not interrupted by gaps anymore, except when CPU is
stalling or in high load.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <kkeil@suse.de>
diff --git a/drivers/isdn/mISDN/dsp.h b/drivers/isdn/mISDN/dsp.h
index 6c3fed6..ded9480 100644
--- a/drivers/isdn/mISDN/dsp.h
+++ b/drivers/isdn/mISDN/dsp.h
@@ -198,6 +198,7 @@
 	/* hardware stuff */
 	struct dsp_features features;
 	int		features_rx_off; /* set if rx_off is featured */
+	int		features_fill_empty; /* set if fill_empty is featured */
 	int		pcm_slot_rx; /* current PCM slot (or -1) */
 	int		pcm_bank_rx;
 	int		pcm_slot_tx;