KVM: arm64: Fix confusion in documentation for pKVM SME assert

As raised in the review comments for the original patch the assert and
comment added in afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are
disabled in protected mode") are bogus. The comments says that we check
that we do not have SME enabled for a pKVM guest but the assert actually
checks to see if the host has anything set in SVCR which is unrelated to
the guest features or state, regardless of if those guests are protected
or not. This check is also made in the hypervisor, it will refuse to run
a guest if the check fails, so it appears that the assert here is
intended to improve diagnostics.

Update the comment to reflect the check in the code, and to clarify that
we do actually enforce this in the hypervisor. While we're here also
update to use a WARN_ON_ONCE() to avoid log spam if this triggers.

Fixes: afb91f5f8ad7 ("KVM: arm64: Ensure that SME controls are disabled in protected mode")
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>