drivers/cc2420: adapt FIFO access to recent msp430 SPI changes

When redoing the SPI driver for the msp430 platforms an assert
statement was introduced to prohibit SPI access without any
buffers. Since in the existing code the FIFO pointer is incremented
through a dummy read this results in triggering aforementioned
assert.

drivers/cc2420/cc2420.c

@@ -217,7 +217,7 @@ int cc2420_rx(cc2420_t *dev, uint8_t *buf, size_t max_len, void *info)
 {
     uint8_t len;
 
-    /* get the packet length (without dropping it) (first byte in RX FIFO */
+    /* get the packet length (without dropping it) (first byte in RX FIFO) */
     cc2420_ram_read(dev, CC2420_RAM_RXFIFO, &len, 1);
     len -= 2;   /* subtract RSSI and FCF */
 
@@ -227,10 +227,13 @@ int cc2420_rx(cc2420_t *dev, uint8_t *buf, size_t max_len, void *info)
 
     /* if a buffer is given, read (and drop) the packet */
     if (buf) {
+        /* We could the drop length byte here, msp430 platforms don't allow
+         * empty reads so we don't do it and read the length byte again. */
+        cc2420_fifo_read(dev, &len, 1);
+        len -=2; /* subtract RSSI and FCF */
+
         len = (len > max_len) ? max_len : len;
 
-        /* drop length byte */
-        cc2420_fifo_read(dev, NULL, 1);
         /* read fifo contents */
         DEBUG("cc2420: recv: reading %i byte of the packet\n", (int)len);
         cc2420_fifo_read(dev, buf, len);