iwlwifi: fix-up botched revert

In the revert of "iwlwifi: move _agn statistics related structure", I
need to use CONFIG_IWLWIFI_DEBUGFS instead of CONFIG_IWLWIFI_DEBUG in
the private structure definition.  Without this patch, it is possible
to get this:

drivers/net/wireless/iwlwifi/iwl-rx.c: In function 'iwl_accumulative_statistics':
drivers/net/wireless/iwlwifi/iwl-rx.c:304: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:305: error: 'struct iwl_priv' has no member named 'delta_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:306: error: 'struct iwl_priv' has no member named 'max_delta'
drivers/net/wireless/iwlwifi/iwl-rx.c:321: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:323: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:325: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:327: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:329: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:331: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c: In function 'iwl_reply_statistics':
drivers/net/wireless/iwlwifi/iwl-rx.c:484: error: 'struct iwl_priv' has no member named 'accum_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:486: error: 'struct iwl_priv' has no member named 'delta_statistics'
drivers/net/wireless/iwlwifi/iwl-rx.c:488: error: 'struct iwl_priv' has no member named 'max_delta'

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>