stmmac: prefetch all dma_erx when use extend_desc

This patch is to prefetch, in the stmmac_rx, the whole
dma_erx descriptor in case of using the extended descriptors.

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