Suspend the chip if possible rather than stop and discard all tx and rx
frames, when changing the mcast list or entering/leaving promiscuous
mode.  Created common pcnet32_suspend routine.

Tested ia32 and ppc64
Signed-off-by: Don Fry <brazilnut@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>