mISDN: Echo canceler now gets delay information from hardware

Added tx-fifo information for calculation of current delay to sync tx and rx
streams for echo canceler.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>