gianfar: Fix skb allocation strategy

gianfar was unable to handle failed skb allocation for rx buffers, so
we were spinning until it succeeded.  Actually, it was worse--we were
spinning for a long time, and then silently failing.  Instead, we take
Stephen Hemminger's suggestion to try the allocation earlier, and drop the
packet if it failed.

We also make a couple of tweaks to how buffer descriptors are set up.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>