selftests: arm64: Fix printf() format mismatch in vec-syscfg

The format for this error message calls for the plain text version of the
error but we weren't supply it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210929151925.9601-2-broonie@kernel.org


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