drivers/at86rf2xx: mask frame length according to data sheet

After some discussion on the devel mailing list about smuggeling in corrupted frames by e.g. SDRs I'd propose we mask the length field according to the datasheet again. It was done before in at86rf2xx_get_length but this funtion did get used anymore along the way.

drivers/at86rf2xx/at86rf2xx_netdev.c

@@ -140,8 +140,8 @@ static int _recv(netdev2_t *netdev, char *buf, int len, void *info)
     /* get the size of the received packet */
     at86rf2xx_fb_read(dev, &phr, 1);
 
-    /* Ignore FCS for packet length */
-    pkt_len = phr - 2;
+    /* ignore MSB (refer p.80) and substract length of FCS field */
+    pkt_len = (phr & 0x7f) - 2;
 
     /* just return length when buf == NULL */
     if (buf == NULL) {