net: sxgbe: rearrange dma descriptor

This patch moves cksum_ctl to tx_rd_des23 from cksum_pktlen for correct checksum
offloading and modifies size for Tx/Rx descriptor.

Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>