stmmac: reorganise class operations.

This patch reorganises the internal stmmac ops structure.
The stmmac_ops has been splitted into other three structures named:
 stmmac_ops
 stmmac_dma_ops
 stmmac_desc_ops

This makes the code more clear and also helps the next work to
make the driver more generic.

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