arm64: cpufeature: Warn if we attempt to read a zero width field

arm64: cpufeature: Warn if we attempt to read a zero width field

Add a WARN_ON_ONCE() when extracting a field if no width is specified. This
should never happen outside of development since it will be triggered with
or without the feature so long as the relevant ID register is present.  If
the warning triggers hope that the field was the standard 4 bits wide and
soldier on.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220307180900.3045812-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>