serial: omap: Fix RTS handling

The OMAP UART ignores MCR[1] (ie., RTS) when in autoRTS mode. This
makes it impossible for either the serial core or userspace to
manually flow control the sender.

Disable autoRTS mode when RTS is lowered and restore the previous
mode when RTS is raised.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>