iwlwifi: fix missing EXPORT_SYMBOL

When compiling without CONFIG_IWLWIFI_DEBUGFS there is a missing
iwl_update_stats symbol. This is fixed by making this function an inline in
the case when CONFIG_IWLWIFI_DEBUGFS is not set due to the hot path in
which it is used.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>