m68k: Remove unused mach_max_dma_address

This information is unused since the discontinuous memory support
has been introduced in 2007.

Fixes: 12d810c1 ("m68k: discontinuous memory support")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Link: https://lore.kernel.org/r/20201009095621.833192-1-laurent@vivier.eu


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>