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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit af86d12c80f10dcd4a57ff7834891890b255087d)