iwlwifi: set default aggregation frame count limit to 31

Multiple MPDUs can be aggregated, transmitted, and finally acknowledged
together using a single BA frame. Block ACK (BA) contains
bitmap size of 64*16 bits so the maximum frame count is 64.
The default value of aggregation frame count suggested by uCode is 31 to
achieve best performance.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>