tty: Define ASYNC_ replacement bits

Prepare for relocating kernel private state bits out of tty_port::flags
field; tty_port::flags field is not atomic and can become corrupted
by concurrent updates. It also suffers from the complication of sharing
in a userspace-visible field which must be masked.

Define new tty_port::iflags field and new, substitute bit definitions
for the former ASYNC_* flags.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>