net: ftgmac100: Ensure tx descriptor updates are visible

The driver must ensure TX descriptor updates are visible
before updating TX pointer and TX clear pointer.

This resolves TX hangs observed on AST2600 when running
iperf3.

Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>