spidernet: move medium variable into card struct

This moves the medium variable into the spidernet card structure.
It renames the GMII_ variables to BCM54XX specific ones.

Signed-off-by: Jens Osterkamp <jens@de.ibm.com>
Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>