hw/block/nvme: Simplify timestamp sum

As the 'timestamp' variable is declared as a 48-bit bitfield,
we do not need to wrap the sum result.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Message-Id: <20201002075716.1657849-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>