ieee802154: constify struct net_device in some operations

Some of ieee802154 operations really shouldn't change passed net_device.
Constify passed argument.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
diff --git a/drivers/ieee802154/fakehard.c b/drivers/ieee802154/fakehard.c
index 4ea93cc..77fbb51 100644
--- a/drivers/ieee802154/fakehard.c
+++ b/drivers/ieee802154/fakehard.c
@@ -43,7 +43,7 @@
  *
  * Return the ID of the PAN from the PIB.
  */
-static u16 fake_get_pan_id(struct net_device *dev)
+static u16 fake_get_pan_id(const struct net_device *dev)
 {
 	BUG_ON(dev->type != ARPHRD_IEEE802154);
 
@@ -58,7 +58,7 @@
  * device. If the device has not yet had a short address assigned
  * then this should return 0xFFFF to indicate a lack of association.
  */
-static u16 fake_get_short_addr(struct net_device *dev)
+static u16 fake_get_short_addr(const struct net_device *dev)
 {
 	BUG_ON(dev->type != ARPHRD_IEEE802154);
 
@@ -78,7 +78,7 @@
  * Note: This is in section 7.2.1.2 of the IEEE 802.15.4-2006
  *       document.
  */
-static u8 fake_get_dsn(struct net_device *dev)
+static u8 fake_get_dsn(const struct net_device *dev)
 {
 	BUG_ON(dev->type != ARPHRD_IEEE802154);
 
@@ -98,7 +98,7 @@
  * Note: This is in section 7.2.1.2 of the IEEE 802.15.4-2006
  *       document.
  */
-static u8 fake_get_bsn(struct net_device *dev)
+static u8 fake_get_bsn(const struct net_device *dev)
 {
 	BUG_ON(dev->type != ARPHRD_IEEE802154);