net/tg3: Avoid delay during MMIO access

When the EEH error is the result of a fenced host bridge, MMIO accesses
can be very slow (milliseconds) to timeout and return all 1's,
thus causing the driver various timeout loops to take way too long and
trigger soft-lockup warnings (in addition to taking minutes to recover).

It might be worthwhile to check if for any of these cases, ffffffff is
a valid possible value, and if not, bail early since that means the HW
is either gone or isolated. In the meantime, checking that the PCI channel
is offline would be workaround of the problem.

Cc: <stable@vger.kernel.org> # v3.0+
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>