HSD #1508890903: net: ethernet: altera: Fixes size of DMA buffer allocation

commit fcb9f187be4cfa2183615aa196f299fff45eac38 from
https://github.com/altera-opensource/linux-socfpga.git

Fixes 'struct altera_dma_private' to 'struct altera_dma_buffer'.

Allocates the buffer based on size of struct altera_dma_buffer instead
of struct altera_dma_private.

Signed-off-by: Patrick Heffernan <pheffernan@t3innovation.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Liwei Song <liwei.song@windriver.com>