net: stmmac: simplify the common DMA init API

Use struct stmmac_dma_cfg *dma_cfg as an argument rather
than using all the struct members as individual arguments.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>