net: l2: ieee802154: Fix a wrong channel change during association

The current association logic starts by changing the channel. The way it
is done is wrong because it dereferences req->channel which is simply
not initialized by the caller. But anyway, the command itself does not
support providing a channel so we must already be on the right one when
trying to associate. Hence, drop this channel change call.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>