ath9k: Fix channel context creation

If a new context is being added in addition to the current one,
then send the ASSIGN event to abort a running scan since
the addition of a context is usually followed by VIF
assignment and further operations.

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