stmmac: rename mac100 as dwmac100 and fix spare coding style

This patch renames the mac100.[ch] as dwmac100.[ch]; this
looks more specific and appropriate for these chip series.
The patch also fixes some spare coding style issues.

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