iwlwifi: Fix error: struct iwl_lq_sta has no member named dbg_fixed_rate

iwlwifi: Fix error: struct iwl_lq_sta has no member named dbg_fixed_rate

While compiling linux-next (next-20101216) I fell over this breakage:
...
drivers/net/wireless/iwlwifi/iwl-agn-rs.c: In function ‘iwl_rs_rate_init’:
drivers/net/wireless/iwlwifi/iwl-agn-rs.c:2876:8: error: ‘struct iwl_lq_sta’ has no member named ‘dbg_fixed_rate’

dbg_fixed_rate is only used when CONFIG_MAC80211_DEBUGFS is set:

[ drivers/net/wireless/iwlwifi/iwl-agn-rs.h ]
...
 #ifdef CONFIG_MAC80211_DEBUGFS
       struct dentry *rs_sta_dbgfs_scale_table_file;
       struct dentry *rs_sta_dbgfs_stats_table_file;
       struct dentry *rs_sta_dbgfs_rate_scale_data_file;
       struct dentry *rs_sta_dbgfs_tx_agg_tid_en_file;
       u32 dbg_fixed_rate;
 #endif

The issue was introduced by commit a1da077bc36368eb7d6312e7e49260f0a3d92c77:
"iwlwifi: clear dbg_fixed_rate during init"

Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>