tg3: Don't modify ip header fields when doing GSO

tg3 uses GSO as workaround if the hardware cannot perform TSO on certain
packets.  We should not modify the ip header fields if we do GSO on the
packet.  It happens to work by accident because GSO recalculates the IP
checksum and IP total length.

Also fix the tg3_start_xmit comment to reflect that this is the only
xmit function for all devices.

Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>