powerpc: Force cast on memcpy_toio()

sparse reports the following warning:

  CHECK   arch/powerpc/cpu/mpc8xx/micropatch_usb_sof.c
arch/powerpc/cpu/mpc8xx/micropatch_usb_sof.c:29:9: warning: cast removes address space '<asn:2>' of expression
arch/powerpc/cpu/mpc8xx/micropatch_usb_sof.c:30:9: warning: cast removes address space '<asn:2>' of expression

This is because of (void *) casts for using memcpy() as a substitute.

Do like other architectures, __force the cast to silence the warning

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>