net: ftgmac100: Ensure tx descriptor updates are visible

[ Upstream commit 4186c8d9e6af57bab0687b299df10ebd47534a0a ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>