net: stmmac: return error if no DMA configuration is found

All drivers except pci glue layer calls stmmac_probe_config_dt.
stmmac_probe_config_dt does a kzalloc dma_cfg.

pci glue layer does kzalloc dma_cfg explicitly, so all current
drivers does a kzalloc dma_cfg.

Return an error if no DMA configuration is found, that way
we can assume that the DMA configuration always exists.

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>