IEEE 802.15.4: remove get_bsn from "struct ieee802154_mlme_ops"

It served no purpose: we never call it from anywhere in the stack
and the only driver that did implement it (fakehard) merely provided
a dummy value.

There is also considerable doubt whether it would make sense to
even attempt beacon processing at this level in the Linux kernel.

Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>