Update em(4) with D3162 after testing further on hardware that failed
to attach with the last version of this commit. This commit fixes
attach failures on "ICH8" class devices via modifications to
e1000_init_nvm_params_ich8lan()

• Fix compiler warning in 80003es2lan.c
• Add return value handler for e1000_*_kmrn_reg_80003es2lan
• Fix usage of DEBUGOUT
• Remove unnecessary variable initializations.
• Removed unused variables (complaints from gcc).
• Edit defines in 82571.h.
• Add workaround for igb hw errata.
• Shared code changes for Skylake/I219 support.
• Remove unused OBFF and LTR functions.

Tested by some of the folks that reported breakage in previous incarnation.
Thanks to AllanJude, gjb, gnn, tijl for tempting fate with their machines.

Submitted by: erj@freebsd.org
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D3162