serial: omap: fix software flow control

Software flow control register bits were not defined correctly.

Also clarify the IXON and IXOFF logic to reflect what userspace wants.

Cc: stable@vger.kernel.org
Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>