nvmecontrol: Create letoh to generically convert to host order

Using _Generic, create letoh which will generically convert uintXX_t
types from little endian to host, regardless of the size. This name has
been floated as a possible addition to endian.h.

Sponsored by: Netflix
Discussed with: jhb
Reviewed by: chuck
Differential Revision: https://reviews.freebsd.org/D44649