tty: serial: Fix bit order in RS485 flag definitions

Since the commit 93f3350 ("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 76ac8e2 ("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: 76ac8e2 ("tty: serial: Cleanup the bit shift with macro")
Fixes: 6056f20 ("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>

Author: cniedermaier

include/uapi/linux/serial.h

@@ -145,12 +145,13 @@ struct serial_rs485 {
 #define SER_RS485_ENABLED		_BITUL(0)
 #define SER_RS485_RTS_ON_SEND		_BITUL(1)
 #define SER_RS485_RTS_AFTER_SEND	_BITUL(2)
-#define SER_RS485_RX_DURING_TX		_BITUL(3)
-#define SER_RS485_TERMINATE_BUS		_BITUL(4)
-#define SER_RS485_ADDRB			_BITUL(5)
-#define SER_RS485_ADDR_RECV		_BITUL(6)
-#define SER_RS485_ADDR_DEST		_BITUL(7)
-#define SER_RS485_MODE_RS422		_BITUL(8)
+/* Placeholder for bit 3: SER_RS485_RTS_BEFORE_SEND, which isn't used anymore */
+#define SER_RS485_RX_DURING_TX		_BITUL(4)
+#define SER_RS485_TERMINATE_BUS		_BITUL(5)
+#define SER_RS485_ADDRB			_BITUL(6)
+#define SER_RS485_ADDR_RECV		_BITUL(7)
+#define SER_RS485_ADDR_DEST		_BITUL(8)
+#define SER_RS485_MODE_RS422		_BITUL(9)
 
 	__u32	delay_rts_before_send;
 	__u32	delay_rts_after_send;