m68k: mac: Don't send uninitialized data in IOP message reply

Clear the message reply before calling iop_complete(). This code path is
not normally executed but should that happen let's arrange for consistent
behaviour from the IOP.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Cc: Joshua Thompson <funaho@jurai.org>
Link: https://lore.kernel.org/r/8e35df4d193b082cb6285b1f30c949ff7e30e99e.1590880333.git.fthain@telegraphics.com.au


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