nvmft: Ignore CC.EN=1 when initiating a shutdown

[jhb]

Summary

If a property set of CC requests both a shutdown (by setting CC.SHN to
a nonzero value) and controller enable (by setting CC.EN=1), give
precedence to the shutdown request and force CC.EN to 0.

Sponsored by: Chelsio Communications

[imp]

Do we need to avoid this on physical controllers to?

[jhb]

In D60471#1392711, @imp wrote:

Do we need to avoid this on physical controllers to?

No, for PCIe controllers this would be in the firmware/ASIC. Note that this stack of change are all on the controller side, so think of it as equivalent to the firmware/ASIC, not nvme(4).