m68k: muldi3: Use semicolon instead of comma

This code works, but it is cleaner to use a semicolon at the end of a
statement instead of a comma.

Extracted from a big anonymous patch by Julia Lawall
<julia.lawall@inria.fr>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20210914073155.3883532-1-geert@linux-m68k.org