e1000e: reformat register test code, fix some minor initialization

The register tests should be run with all the proper flags enabled
to maximize the test coverage code and make sure we are as close
as we can get to testing regular traffic.

Reformat the code for readability. Minor cleanups in the descriptor
ring setup.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>