HAL_ChibiOS: fixed DMA on SPI on H743

this fixes DMA failures affecting IMUs on H743. The reason for the
failure is not yet clear, but this reverts back to our old stragegy of
always using the bouncebuffer on H7 as a quick workaround

libraries/AP_HAL_ChibiOS/hwdef/common/malloc.c

@@ -631,6 +631,10 @@ bool mem_is_dma_safe(const void *addr, uint32_t size, bool filesystem_op)
 #else
     uint32_t flags = MEM_REGION_FLAG_DMA_OK;
 #if defined(STM32H7)
+    if (!filesystem_op) {
+        // use bouncebuffer for all non FS ops on H7
+        return false;
+    }
     if (((uint32_t)addr) & 0x1F) {
         return false;
     }