forcedeth: checksum fix

The driver should inform the stack when checksum has been performed by
the HW when both IP and TCP (or UDP) checksum flags are indicated by HW.

Previously, it would also inform the stack when only IP checksum flag
was indicated by HW. This can cause data corruption when IP fragments
are used. The IP Identification field can wrap around and cause data
from new fragments to fill into older fragment slots with same IP Id.
The stack would then not perform TCP/UDP checksum (after re-assembly of
all fragments) since driver falsely stated it was already calculated.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>