ath10k: Fix un-initialized debug objects.

If the 'ar' registration fails early, the debug
objects were not initialized, and trouble ensued when
the object was later destroyed and the un-initialized
objects were accessed.

Per Kalle's suggestion, move the debug_destroy to the
core_unregister method.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>