tests/exynos: replace return by break

The 'usage' function already does exit(0), so that this
'return -EINVAL' is never called. Just put a break there
to avoid confusion.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>