bhyve: Address -Wunused* warnings in pci_nvme.c

Currently these are not reported because bhyve is compiled with WARNS=2.
Let's start taking small steps towards enabling more warnings.

No functional change intended.

Reviewed by: chuck, imp, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36118