arm64: capabilities: Handle sign of the feature bit

commit ff96f7bc7bf6393eef8ff2bde1279715ce13343a usptream

Use the appropriate accessor for the feature bit by keeping
track of the sign of the feature.

This is a pre-requisite for the commit 28c5dcb22f90 upstream,
which fixes the arm64_ftr_value() for signed feature fields.

Cc: stable@vger.kernel.org # v4.4
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>