nvme: removed unused 'regs' variable

[ziaee]

Summary

The private struct has a 'regs' member that's only written to
and otherwise unused. This change removes it.

Proxying on behalf of Matt Delco <[e-mail]>

Requested by: jhb

[imp]

Yes. We only use struct nvme_registers for register offsets, and otherwise use bus space to get the data.

[jhb]

Great that we already use bus_space. I feared the worst when I saw this assignment fly by in the earlier diff.