selftests: arm64: Remove bogus error check on writing to files

Due to some refactoring with the error handling we ended up mangling things
so we never actually set ret and therefore shouldn't be checking it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210929151925.9601-3-broonie@kernel.org


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