omap-serial: Allow IXON and IXOFF to be disabled.

Fixes logic bug that software flow control cannot be disabled, because
serial_omap_configure_xonxoff() is not called if both IXON and IXOFF bits
are cleared.

Signed-off-by: Nick Pelly <npelly@google.com>
Acked-by: Govindraj.R <govindraj.raja@ti.com>
Tested-by: Govindraj.R <govindraj.raja@ti.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>