isdn: fix strlen() usage

There was a missing "else" statement so the original code overflowed if
->master->name was too long.  Also the ->slave and ->master buffers can
hold names with 9 characters and a NULL so I cleaned it up to allow
another character.

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