net: ethernet: stmicro: stmmac: generate stmmac dma conf before open

Rework the driver to generate the stmmac dma_conf before stmmac_open.
This permits a function to first check if it's possible to allocate a
new dma_config and then pass it directly to __stmmac_open and "open" the
interface with the new configuration.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>