serial: uapi: Add support for bus termination

The Siemens IOT2040 comes with a RS485 interface that allows to enable
or disable bus termination via software. Add a bit to the flags field of
serial_rs485 that applications can set in order to request this feature
from the hardware. This seems generic enough to add it for everyone.
Existing driver will simply ignore it when set.

Signed-off-by: Sascha Weisenberger <sascha.weisenberger@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/uapi/linux/serial.h

@@ -122,6 +122,9 @@ struct serial_rs485 {
 #define SER_RS485_RTS_AFTER_SEND	(1 << 2)	/* Logical level for
 							   RTS pin after sent*/
 #define SER_RS485_RX_DURING_TX		(1 << 4)
+#define SER_RS485_TERMINATE_BUS		(1 << 5)	/* Enable bus
+							   termination
+							   (if supported) */
 	__u32	delay_rts_before_send;	/* Delay before send (milliseconds) */
 	__u32	delay_rts_after_send;	/* Delay after send (milliseconds) */
 	__u32	padding[5];		/* Memory is cheap, new structs