net: ethernet: stmicro: stmmac: move dma conf to dedicated struct

Move dma buf conf to dedicated struct. This in preparation for code
rework that will permit to allocate separate dma_conf without affecting
the priv struct.

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