net: mac802154: comparision issue of type cast, finding by EXTRA_CFLAGS=-W

Change MAC802154_CHAN_NONE from ~(u8)0 to 0xff, or the comparison in
mac802154_wpan_xmit() for ``chan == MAC802154_CHAN_NONE'' will not
succeed.

This bug can be boiled down to ``u8 foo = 0xff; if (foo == ~(u8)0)
[...] else [...]'' where the condition will always take the else
branch.
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac802154/mac802154.h

@@ -90,7 +90,7 @@ struct mac802154_sub_if_data {
 
 #define MAC802154_MAX_XMIT_ATTEMPTS	3
 
-#define MAC802154_CHAN_NONE		(~(u8)0) /* No channel is assigned */
+#define MAC802154_CHAN_NONE		0xff /* No channel is assigned */
 
 extern struct ieee802154_reduced_mlme_ops mac802154_mlme_reduced;
 extern struct ieee802154_mlme_ops mac802154_mlme_wpan;