ser_gigaset: fix up NULL checks

Commit f34d7a5b ("tty: The big operations rework") changed
tty->driver to tty->ops but left NULL checks for tty->driver untouched.
Fix.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
[pebolle: removed Fixes tag]
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>