tty: actually undefine superseded ASYNC flags

tty: actually undefine superseded ASYNC flags

Some kernel-internal ASYNC flags have been superseded by tty-port flags
and should no longer be used by kernel drivers.

Fix the misspelled "__KERNEL__" compile guards which failed their sole
purpose to break out-of-tree drivers that have not yet been updated.

Fixes: 5c0517fefc92 ("tty: core: Undefine ASYNC_* flags superceded by TTY_PORT* flags")
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407095208.31838-2-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>