selftests: arm64: Fix and enable test for setting current VL in vec-syscfg

We had some test code for verifying that we can write the current VL via
the prctl() interface but the condition for the test was inverted which
wasn't noticed as it was never actually hooked up to the array of tests
we execute. Fix this.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210929151925.9601-4-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>