serial: 8250_omap: Use UPSTAT_AUTORTS for RTS handling

Commit 88838d3112702 ("serial: omap_8250: Fix RTS handling") fixed
RTS pin control when in autoRTS mode.

New support added in "serial: core: Rework hw-assisted flow control support"
enables a much simpler approach; rather than masking out autoRTS
whenever writing the EFR register, use the UPSTAT_* mode to determine if
autoRTS should be enabled when raising RTS (in omap8250_set_mctrl()).

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