myri10ge: move data structures into a single slice

To prepare and simplify multislice rx support, add a single slice
structure and move some fields in there.
No functional change yet.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Andrew Gallatin <gallatin@myri.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>