[PATCH] pcnet32: Prevent hang with 79c976

Some boards using the 79c976 pcnet32 chip will hang the system if the
ethtool --register-dump is performed with the device operational.  The
request to read bcr30 is retried by the PCI device infinitely without
returning data, hanging the system.

Tested ia32 and ppc64.

Signed-off-by: Don Fry <brazilnut@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>