m68k/mac: Clarify IOP message alloc/free confusion

The alloc/free metaphor used for IOP messages is misleading and can
cause mistakes like the pointless msg2 temporary variable. Use a more
meaningful name to help simplify the code.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>