myri10ge: add multislices support

Add multi-slice/MSI-X support. By default, a single slice
(and the normal firmware) are used.  To enable msi-x, multi-slice
mode, one must load the driver with myri10ge_max_slices set to
either -1, or something larger than 1.
Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Andrew Gallatin <gallatin@myri.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>