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/include/linux/mISDNif.h b/include/linux/mISDNif.h
index 8f2d60d..74c903c 100644
--- a/include/linux/mISDNif.h
+++ b/include/linux/mISDNif.h
@@ -312,6 +312,7 @@
 #define MISDN_CTRL_SETPEER		0x0040
 #define MISDN_CTRL_UNSETPEER		0x0080
 #define MISDN_CTRL_RX_OFF		0x0100
+#define MISDN_CTRL_FILL_EMPTY		0x0200
 #define MISDN_CTRL_HW_FEATURES_OP	0x2000
 #define MISDN_CTRL_HW_FEATURES		0x2001
 #define MISDN_CTRL_HFC_OP		0x4000