ath9k: Enable multi-channel properly

In MCC mode, currently the decision to enable
the multi-channel state machine is done
based on the association status if one of
the interfaces assigned to a context is in
station mode.

This allows the driver to switch to the other
context before the current station is able to
complete the 4-way handshake in case it is
required and this causes problems.

Instead, enable multi-channel mode when the
station moves to the authorized state. This
disallows an early switch to the other channel.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>