iwlagn: dbg_fixed_rate only used when CONFIG_MAC80211_DEBUGFS enabled

Fix compiling error when CONFIG_MAC80211_DEBUGFS is not enabled
drivers/net/wireless/iwlwifi/iwl-agn-rs.c:351: error: 'struct iwl_lq_sta' has no member named 'dbg_fixed_rate'
drivers/net/wireless/iwlwifi/iwl-agn-rs.c:1076: error: 'struct iwl_lq_sta' has no member named 'dbg_fixed_rate'

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>