stmmac: fix memory barriers

Fix up memory barriers in stmmac driver. They are meant to protect
against DMA engine, so smp_ variants are certainly wrong, and dma_
variants are preferable.

Signed-off-by: Pavel Machek <pavel@denx.de>
Tested-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>