ppc440_uc: Fix a copy/paste error

Missed in 3c409c19, hopefully reported by Coverity.

Fixes: Coverity CID 1393788 (Copy-paste error)
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NBALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/ppc440_uc.c

@@ -935,7 +935,7 @@ static void dcr_write_dma(void *opaque, int dcrn, uint32_t val)
                     if (wptr) {
                         cpu_physical_memory_unmap(wptr, wlen, 1, didx);
                     }
-                    if (wptr) {
+                    if (rptr) {
                         cpu_physical_memory_unmap(rptr, rlen, 0, sidx);
                     }
                 }