A controller that will not clear CSTS.RDY after CC.EN is cleared leaves
nvme_ctrlr_hw_reset() with nothing left to try, so the reset fails and
the controller is marked failed. Controllers that set CAP.NSSRS accept
a write of "NVMe" to NSSR.NSSRC, which resets the whole NVM subsystem
and is a stronger reset than a Controller Level Reset.

Try it once, then retry the disable. The subsystem reset drops every
PCIe link in the subsystem to Detect, so the PCI register space is saved
and restored around it.

Refer to NVM Express Base Specification, Revision 2.4, section 3.7.1 and
Figure 43.