ser_gigaset: turn nonsense checks into WARN_ON

These checks do nothing useful to protect the code from races. On the
other hand if the old code has been masking a real bug we would like to
know about it.

The check for tiocmset is kept because it is valid for a tty driver to
have a NULL tiocmset method. That in itself is probably a mistake given
modern coding practices - but needs fixing in the tty layer.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>