mISDN: avmfritz use the bigger fifo of chip version 2

If we detect the latest hardware revision we should use the bigger fifo
to avoid TX underruns and have less interrupts.
TX underruns should be logged as warning.

Signed-off-by: Karsten Keil <kkeil@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>