TTY: isdn, remove ISDN_ASYNC_* flags

They are the same as TTY ones. So there is no need to redefine them.
Remove ISDN_ASYNC_* and use only ASYNC_*. Except the MAGIC number, of
course.

While we are there, remove also the SERIAL_TYPE flags which are
unused.

Perhaps we should move the ASYNC flags from serial.h to tty.h given
they are used by the tty layer and tty drivers, not only serial?

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>