tty: serial: 21285: stop using the unused[] variable from struct uart_port

Much like the samsung_tty driver (now I know where they copied the idea
from), the 21285 uart driver uses 2 bytes from the "unused" array of
struct uart_port to keep tx/rx enabled/disabled state.  Those fields are
going away (they were never really needed in the first place), so fix up
the 21285 driver by another horrible hack.

Instead of creating a whole structure for just 2 bytes, just use two
bits from the private_data pointer instead, as that pointer is never
used.  The two bits reflect if tx/rx is now enabled/disabled.

Astute readers will note that once rx is disabled, nothing ever seems to
turn it back on, making one wonder if anyone has ever done this.

Reported-by: kbuild test robot <lkp@intel.com>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20191219145109.GA1962496@kroah.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>