stmmac: add memory barriers at appropriate places

This patch, provided by ST SPEAr developers,
has fixed a problem raised on ARM CA9 where
happened that the dma_transmission was enabled before
the dma descriptors were properly filled. To guarantee this
data memory barriers have been explicity used in the driver.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>