net: stmmac: Uniformize the use of dma_init_* callbacks

Instead of relying on the GMAC version for choosing if we need to use
dma_init or dma_init_{rx/tx}_chan callback, lets uniformize this and
always use the dma_init_{rx/tx}_chan callbacks.

While at it, fix the use of dma_init_chan callback, which shall be
called for as many channels as the max of rx/tx channels.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Vitor Soares <soares@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>