[POWERPC] of_platform_make_bus_id(): make `magic' int

of_platform_make_bus_id(): Kill a compiler warning which is a real
bug on PPC64 by changing `magic' to `int'.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>