[ISDN] sc: Fix sndpkt to have the correct number of arguments

isdn_if.writebuf_skb has an additional ack flag argument which
was missing from sndpkt leading to the following warning:
  CC [M]  drivers/isdn/sc/init.o
drivers/isdn/sc/init.c: In function ‘sc_init’:
drivers/isdn/sc/init.c:281: warning: assignment from incompatible pointer type

Note that this doesn't actually do anything with the flag, it
just fixes the warning (and probably accessing the last argument).

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: David S. Miller <davem@davemloft.net>