atl1c: update right threshold for TSO

atl1c_configure_tx used a wrong value of MAX_TX_OFFLOAD_THRESH(9KB)
for TSO threshold.
the right value should be 7KB
Fast Ethernet controller doesn't support Jumbo frame.

Signed-off-by: xiong <xiong@qca.qualcomm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>