arm64/ptrace: Fix typoes in sve_set() comment

The ptrace trace SVE flags are prefixed with SVE_PT_*. Update the
comment accordingly.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>