staging: greybus: loop_backtest: fixed consistent spacing style issue

Fixed incosistent spacing around arithmetic operator.

Signed-off-by: Jonathan Bowie <eudjtb@gmail.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>