net: mv643xx_eth: Avoid setting the initial TCP checksum

As specified in the datasheet, the driver can set the "L4Chk_Mode" flag
(bit 10) in the Tx descriptor command/status to specify that a frame is not
IP fragmented and that the controller is in charge of generating the TCP/IP
checksum. This must be used together with the "GL4chk" flag (bit 17).

These two flags allow to avoid setting the initial TCP checksum in the l4i_chk
field of the Tx descriptor, which is needed to support software TSO.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>