serial_core: Handle TIOC[GS]RS485 ioctls.

The following drivers: 8250_core, atmel_serial, max310x, mcf, omap-serial
and sci16is7xx implement code to handle RS485 ioctls.

In order to avoid code duplication, we implement a simple ioctl handler
on the serial_core layer.

This handler can be used by all the other drivers instead of duplicating
code.

Until this is the only RS485 ioctl handler, it will try first the
rs485_config callback and if it is not present it will call the driver
specific ioctl.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 40b4cc4..3231a43 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -131,6 +131,8 @@
 	void			(*pm)(struct uart_port *, unsigned int state,
 				      unsigned int old);
 	void			(*handle_break)(struct uart_port *);
+	int			(*rs485_config)(struct uart_port *,
+						struct serial_rs485 *rs485);
 	unsigned int		irq;			/* irq number */
 	unsigned long		irqflags;		/* irq flags  */
 	unsigned int		uartclk;		/* base uart clock */
@@ -231,6 +233,7 @@
 	unsigned char		unused[2];
 	struct attribute_group	*attr_group;		/* port specific attributes */
 	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
+	struct serial_rs485     rs485;
 	void			*private_data;		/* generic platform data pointer */
 };