HAL_ChibiOS: adjust dma reserve allocation

use larger target and allow for smaller allocation

libraries/AP_HAL_ChibiOS/hwdef/common/malloc.c

@@ -59,7 +59,7 @@ static memory_heap_t heaps[NUM_MEMORY_REGIONS];
 
 // size of memory reserved for dma-capable alloc
 #ifndef DMA_RESERVE_SIZE
-#define DMA_RESERVE_SIZE 4096
+#define DMA_RESERVE_SIZE 6144
 #endif
 
 #if DMA_RESERVE_SIZE != 0
@@ -81,9 +81,15 @@ void malloc_init(void)
       create a DMA reserve heap, to ensure we keep some memory for DMA
       safe memory allocations
      */
-    void *dma_reserve = malloc_dma(DMA_RESERVE_SIZE);
+    uint32_t reserve_size = DMA_RESERVE_SIZE;
-    osalDbgAssert(dma_reserve != NULL, "DMA reserve");
+    while (reserve_size > 0) {
-    chHeapObjectInit(&dma_reserve_heap, dma_reserve, DMA_RESERVE_SIZE);
+        void *dma_reserve = malloc_dma(reserve_size);
+        if (dma_reserve != NULL) {
+            chHeapObjectInit(&dma_reserve_heap, dma_reserve, reserve_size);
+            break;
+        }
+        reserve_size = (reserve_size * 7) / 8;
+    }
 #endif
 }