nvme: Firmware revisions in the firmware slot info logpage are ASCII strings

In particular, don't try to byteswap the values as 64-bit integers and
always print a non-empty version as a string.

Reviewed by: chuck, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44121

(cherry picked from commit 7485926e09a08fbfe83a9bc908e7d43aaca4c172)