hw/ppc/ppc405_uc.c: Avoid integer overflows

hw/ppc/ppc405_uc.c: Avoid integer overflows

When performing clock calculations, the ppc405_uc code
has several places where it multiplies together two
32-bit variables and assigns the result to a 64-bit
variable. This doesn't quite do what is intended because
C will compute a 32-bit multiply result. Add casts to
ensure we don't truncate the result.

(Spotted by Coverity, CID 1005504, 1005505.)

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: David Gibson <[email protected]>