USB: serial: kobil_sct: log failure to update line settings

Log failure to update the line settings in set_termios().

This also avoids a W=1 (-Wunused-but-set-variable) warning.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>