iwlwifi: Fix unconditional access to station->tidp[].agg.

Reportred by Ingo Molnar:

drivers/net/wireless/iwlwifi/iwl-debugfs.c: In function 'iwl_dbgfs_stations_read':
drivers/net/wireless/iwlwifi/iwl-debugfs.c:256: error: 'struct iwl4965_tid_data' has no member named 'agg'

Needs CONFIG_IWL4965_HT protection.

Signed-off-by: David S. Miller <davem@davemloft.net>