serial: omap: Fix EFR write on RTS deassertion

BugLink: http://bugs.launchpad.net/bugs/1744794

commit 2a71de2f upstream.

Commit 348f9bb3 ("serial: omap: Fix RTS handling") sought to enable
auto RTS upon manual RTS assertion and disable it on deassertion.
However it seems the latter was done incorrectly, it clears all bits in
the Extended Features Register *except* auto RTS.

Fixes: 348f9bb3 ("serial: omap: Fix RTS handling")
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/tty/serial/omap-serial.c

@@ -693,7 +693,7 @@ static void serial_omap_set_mctrl(struct uart_port *port, unsigned int mctrl)
 	if ((mctrl & TIOCM_RTS) && (port->status & UPSTAT_AUTORTS))
 		up->efr |= UART_EFR_RTS;
 	else
-		up->efr &= UART_EFR_RTS;
+		up->efr &= ~UART_EFR_RTS;
 	serial_out(up, UART_EFR, up->efr);
 	serial_out(up, UART_LCR, lcr);