[PATCH] pcnet32: Cleanup rx buffers after loopback test.

More cleanup to pcnet32_loopback_test to release receive buffers if
device is not up.  Created common routine to free rx buffers.

Tested ia32 and ppc64

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