nda(4): Remove unnecessary union and avoid Clang -Wsizeof-array-divwarning

Clang trunk recently gained this new warning, and complains about the
sizeof(trim->data) / sizeof(struct nvme_dsm_range) expression, since
the left hand side's element type (char) does not match the right hand
side's type. The byte buffer is unnecessary so we can remove it to clean
up the code and fix the warning at the same time.

No functional change.

Submitted by: James Clarke <jrtc27@jrtc27.com>
Reviewed by: imp
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21912