net: sgi: ioc3-eth: refactor rx buffer allocation

Move common code for rx buffer setup into ioc3_alloc_skb and deal
with allocation failures. Also clean up allocation size calculation.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>