ath9k: Fix crash in MCC mode

When a channel context is removed, the hw_queue_base
is set to -1, this will result in a panic because
ath9k_chanctx_stop_queues() can be called on an interface
that is not assigned to any context yet - for example,
when trying to scan.

Fix this issue by setting the hw_queue_base to zero
when a channel context is removed.

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