stmmac: split core and dma for the mac10/100

The patch splits core and dma parts for the mac10/100 device.
This was already done for the GMAC device.
It should make more flexible the driver to support other chips.

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