Use testbit like in tty subsystem for TTY_IO_ERROR testing.

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>