drivers/net/wireless/iwlwifi/iwl-debugfs.c: fix another '%llu' warning

drivers/net/wireless/iwlwifi/iwl-debugfs.c: In function ‘iwl_dbgfs_stations_read’:
drivers/net/wireless/iwlwifi/iwl-debugfs.c:247: warning: format ‘%llu’ expects type ‘long long unsigned int’, but argument 4 has type ‘u64’

Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>