e1000: factor out checking for auto-negotiation availability

Also fix minor indentation issues in the surrounding code.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>


