tg3: Fix TSO loopback test

Commit bb158d69, entitled
"tg3: Add TSO loopback test", mistakenly inverted the checksum field
test from the receive BD.  This patch corrects the problem.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>