e1000: Shared code updates

• 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.

Differential Revision: https://reviews.freebsd.org/D3162
Submitted by: erj
MFC after: 1 month
Sponsored by: Intel Corporation