tty: serial: Fix bit order in RS485 flag definitions

Since the commit 93f3350c ("RS485: fix inconsistencies in the
meaning of some variables"), the definition for bit 3 has been removed.
But with the switch to bit shift macros in commit 76ac8e29

 ("tty:
serial: Cleanup the bit shift with macro"), this gap wasn't preserved.
To avoid a break in user/kernel api of the system skip bit 3 again and
add a placeholder comment.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Fixes: 76ac8e29 ("tty: serial: Cleanup the bit shift with macro")
Fixes: 6056f20f

 ("tty: serial: Add RS422 flag to struct serial_rs485")
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Crescent CY Hsieh <crescentcy.hsieh@moxa.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Lino Sanfilippo <l.sanfilippo@kunbus.com>
Cc: Hugo Villeneuve <hugo@hugovil.com>
Link: https://lore.kernel.org/r/20240119113516.2944-1-cniedermaier@dh-electronics.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>