hw/block/nvme: fix 64 bit register hi/lo split writes

64 bit registers like ASQ and ACQ should be writable by both a hi/lo 32
bit write combination as well as a plain 64 bit write. The spec does not
define ordering on the hi/lo split, but the code currently assumes that
the low order bits are written first. Additionally, the code does not
consider that another address might already have been written into the
register, causing the OR'ing to result in a bad address.

Fix this by explicitly overwriting only the low or high order bits for
32 bit writes.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>