cc2420: Remove netdev access from set_pan

This write access is only required when a modification to the PAN ID
happened directly via this function and not via a netdev::set operation.
The only direct call was done in the reset function of the driver

drivers/cc2420/cc2420_getset.c

@@ -114,7 +114,6 @@ uint16_t cc2420_get_pan(cc2420_t *dev)
 
 void cc2420_set_pan(cc2420_t *dev, uint16_t pan)
 {
-    dev->netdev.pan = pan;
     cc2420_ram_write(dev, CC2420_RAM_PANID, (uint8_t *)&pan, 2);
 }