serial/atmel: Use the rs485 functions on serial_core

In order to unify all the rs485 ioctl handling.
Use the implementation of TIOC[GS]RS485 ioctl handling on serial_core.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>