drivers: net: xgene: Fix MSS programming

Current driver programs static value of MSS in hardware register for TSO
offload engine to segment the TCP payload regardless the MSS value
provided by network stack.

This patch fixes this by programming hardware registers with the
stack provided MSS value.

Since the hardware has the limitation of having only 4 MSS registers,
this patch uses reference count of mss values being used.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>