nvme: Rename NVMEB helper macro to NVMEM

The current macro always builds a full mask for a named field, so use
the M suffix for mask.

Reviewed by: chuck, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D43601

(cherry picked from commit 1dade1f255ee535ad357211395b46188bece52dc)