iwlwifi: set default aggregation frame limit to 63

This gives much better performance at fast 3x3 rates (up to ~160 Mbps).
The scheduler will still make most decisions about batch size based on
available packets and RX parameters.

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