m68k: mac: Refactor iop_preinit() and iop_init()

The idea behind iop_preinit() was to put the SCC IOP into bypass mode.
However, that remains unimplemented and implementing it would be
difficult. Let the comments and code reflect this. Even if iop_preinit()
worked as described in the comments, it gets called immediately before
iop_init() so it might as well part of iop_init().

Cc: Joshua Thompson <funaho@jurai.org>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Link: https://lore.kernel.org/r/0a7b09f5e5f48e270b82041c19e8f20f54c69216.1605847196.git.fthain@telegraphics.com.au


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