e1000: Lock nvm print sysctl

Otherwise results in KASSERT with debug kernels because we rely on the
iflib CTX lock to implement the software serialization to the NVM model

Reviewed by: gallatin
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D32333

(cherry picked from commit 9b3e252e59c6e63594fb20e3f65188dab9e1eeff)