serial: omap: Fix EFR write on RTS deassertion

serial: omap: Fix EFR write on RTS deassertion

BugLink: http://bugs.launchpad.net/bugs/1744213
commit 2a71de2f7366fb1aec632116d0549ec56d6a3940 upstream.

Commit 348f9bb31c56 ("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: 348f9bb31c56 ("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>