have nvme_poll return the status bigs from the completion ring

have nvme_poll return the status bigs from the completion ring
entry. if i ever implement timeouts ill use high bits in the int
or the phase bit to indicate non chip related errors. a successful
chip status conveniently maps to 0. how handy.

this lets me move the completion handling for the controller
identification commands back into the caller. at the moment im just
printing out controller and firmware details like we do on
mfi/mpii/mfii.