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>