hw/nvme: fix missing check for PMR capability

Qiang Liu reported that an access on an unknown address is triggered in
memory_region_set_enabled because a check on CAP.PMRS is missing for the
PMRCTL register write when no PMR is configured.

Cc: qemu-stable@nongnu.org
Fixes: 75c3c9de ("hw/block/nvme: disable PMR at boot up")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/362


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