net-next: ax88796: Add block_input/output hooks to ax_plat_data

Add platform specific hooks for block transfer reads/writes of packet
buffer data, superseding the default provided ax_block_input/output.
Currently used for m68k Amiga XSurf100.

Signed-off-by: Michael Karcher <kernel@mkarcher.dialup.fu-berlin.de>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>