isdn: cleanup: make buffer smaller

This showed up in my audit because we use strcpy() to copy "ds" into a
32 character buffer inside the isdn_tty_dial() function.  But it turns
out that we only ever use the first 32 characters so it's OK.  I have
changed the declaration to make the static checkers happy.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>