TTY: deprecate linux/generic_serial.h

Since nobody in the kernel includes that file, let us remove the
structs visible to the kernel.

However since the userspace sees the file, it still may include that.
hence deprecate the use of the header by an added cpp #warning.

We should remove the file completely after a couple of years.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>