hw/nvme: clean up CC register write logic

hw/nvme: clean up CC register write logic

The SRIOV series exposed an issued with how CC register writes are
handled and how CSTS is set in response to that. Specifically, after
applying the SRIOV series, the controller could end up in a state with
CC.EN set to '1' but with CSTS.RDY cleared to '0', causing drivers to
expect CSTS.RDY to transition to '1' but timing out.

Clean this up.

Reviewed-by: Łukasz Gieryk <lukasz.gieryk@linux.intel.com>
Reviewed-by: Lukasz Maniak <lukasz.maniak@linux.intel.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>