iwlagn: fix aggregation queue scheduler setup

iwlagn's hardware scheduler needs to be set up
with the right aggregation frame limit and
buffer sizes. To achieve this, we need to move
the hardware queue setup to when the session
becomes operational.

Tested-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>