iwlwifi: transport: track number of allocated queues

As the transport will decide how many queues (and MSI-X vectors)
to allocate, add a field to indicate that to the op-mode so it
can size/allocate its own data structures appropriately.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>