MFC ixgbe commits for 10.2:

MFC ixgbe commits for 10.2:

- r280182 - Split the driver into independent pf/vf loadables
- r280197 - Resolve build issues
- r280204 - Fix multiple same-name devclasses
- r280228 - Fix i386 LINT build issues / remove unused variable
- r280252 - Fix building ixgbe with gcc
- r280962 - Make changes to busdma code similar to r257541
- r281772 & r281773 - Remove unused variable
- partial r282280 - stats counter update (ix-only)
- r282289 - Add X550 support
- r282290 - Add X550 makefile updates
- r282293 - Add ixgbe_x550.c to conf/files
- r282299 - Fix gcc compile (extraneous extern declaration)

Finally, add ix_txrx.c to conf/files because it's required for compile in stable/10.

Approved by:	jfv (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/10@283620 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f