iwlwifi: fix build error for CONFIG_IWLAGN=n

drivers/net/wireless/iwlwifi/iwl-rx.c: In function 'iwl_good_ack_health':
drivers/net/wireless/iwlwifi/iwl-rx.c:647: error: 'struct iwl_priv' has no member named '_agn'

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