PPC: avoid function pointer type mismatch, spotted by clang

PPC: avoid function pointer type mismatch, spotted by clang

Fixes clang errors:
  CC    ppc-softmmu/translate.o
/src/qemu/target-ppc/translate.c:3748:13: error: comparison of distinct pointer types ('void (*)(void *, int, int)' and 'void *')
        if (likely(read_cb != SPR_NOACCESS)) {
/src/qemu/target-ppc/translate.c:3748:28: note: instantiated from:
        if (likely(read_cb != SPR_NOACCESS)) {
/src/qemu/target-ppc/translate.c:3903:13: error: comparison of distinct pointer types ('void (*)(void *, int, int)' and 'void *')
        if (likely(write_cb != SPR_NOACCESS)) {
/src/qemu/target-ppc/translate.c:3903:29: note: instantiated from:
        if (likely(write_cb != SPR_NOACCESS)) {

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>