Fix istallion build failure

Commits b02f5ad6 ("istallion: use
tty_port") and a352def2 ("tty: Ldisc
revamp") broke the istallion driver.

Fix the compile error and silence a warning.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>