net: sgi: ioc3-eth: use defines for constants dealing with desc rings

Descriptor ring sizes of the IOC3 are more or less fixed size. To
make clearer where there is a relation to ring sizes use defines.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>