Fix cc2530 DMA priority define values

Fixes #156

cpu/cc253x/dev/dma.c

@@ -87,7 +87,7 @@ dma_reset(uint8_t c)
   dma_conf[c].len_h = 0;
   dma_conf[c].len_l = 1;
   dma_conf[c].wtt = DMA_BLOCK;
-  dma_conf[c].inc_prio = DMA_PRIO_GUARANTEED;
+  dma_conf[c].inc_prio = DMA_PRIO_ASSURED;
   DMA_TRIGGER(c); /** The operation order is important */
   DMA_ARM(c);
   while(DMAARM & (1 << c));

cpu/cc253x/dev/dma.h

@@ -95,9 +95,8 @@
 /* Descriptor Byte 7, Bits[3:0] */
 #define DMA_IRQ_MASK_ENABLE  0x08
 #define DMA_MODE_7_BIT       0x04
-#define DMA_PRIO_HIGHEST     0x03
 #define DMA_PRIO_HIGH        0x02
-#define DMA_PRIO_GUARANTEED  0x01
+#define DMA_PRIO_ASSURED     0x01
 #define DMA_PRIO_LOW         0x00
 
 /** DMA configuration structure */