kselftest/arm64: Fix check for setting new VLs in sve-ptrace

kselftest/arm64: Fix check for setting new VLs in sve-ptrace

The check that the new vector length we set was the expected one was typoed
to an assignment statement which for some reason the compilers didn't spot,
most likely due to the macros involved.

Fixes: a1d7111257cd ("selftests: arm64: More comprehensively test the SVE ptrace interface")
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Dev Jain <dev.jain@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250609-kselftest-arm64-ssve-fixups-v2-1-998fcfa6f240@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>