pasemi_mac: rework ring management

pasemi_mac: rework ring management

Rework ring management, switching to an opaque ring format instead of
the struct-based descriptor+pointer setup, since it will be needed for
SG support.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>