[nvmecontrol] Fix type signedness warning-to-error on gcc-6.4

This fixes a type signedness comparison warning-to-error on
gcc-6.4. The ternary operation casts it right but the actual
assignment doesn't.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D26791

(cherry picked from commit 44c52406ced4cbba704f9bec588a8238d5a5ef32)