[PATCH] pcnet32: Handle memory allocation failures cleanly when resizing tx/rx rings

Fix pcnet32_set_ringparam to handle memory allocation errors without
leaving the adapter in an inoperative state and null pointers waiting to
be dereferenced.

Tested ia32 and ppc64.

Signed-off-by:  Don Fry <brazilnut@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>