wifi: rtw88: fix uninitialized use of primary channel index

clang reports uninitialized use:

>> drivers/net/wireless/realtek/rtw88/main.c:731:2: warning: variable
   'primary_channel_idx' is used uninitialized whenever switch default is
   taken [-Wsometimes-uninitialized]
           default:
           ^~~~~~~
   drivers/net/wireless/realtek/rtw88/main.c:754:39: note: uninitialized
   use occurs here
           hal->current_primary_channel_index = primary_channel_idx;
                                                ^~~~~~~~~~~~~~~~~~~
   drivers/net/wireless/realtek/rtw88/main.c:687:24: note: initialize the
   variable 'primary_channel_idx' to silence this warning
           u8 primary_channel_idx;
                                 ^
                                  = '\0'

This situation could not happen, because possible channel bandwidth
20/40/80MHz are enumerated.

Fixes: 341dd1f7 ("wifi: rtw88: add the update channel flow to support setting by parameters")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20220815062004.22920-1-pkshih@realtek.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/realtek/rtw88/main.c

@@ -697,6 +697,7 @@ void rtw_update_channel(struct rtw_dev *rtwdev, u8 center_channel,
 
 	switch (bandwidth) {
 	case RTW_CHANNEL_WIDTH_20:
+	default:
 		primary_channel_idx = RTW_SC_DONT_CARE;
 		break;
 	case RTW_CHANNEL_WIDTH_40:
@@ -728,8 +729,6 @@ void rtw_update_channel(struct rtw_dev *rtwdev, u8 center_channel,
 			cch_by_bw[RTW_CHANNEL_WIDTH_40] = center_channel - 4;
 		}
 		break;
-	default:
-		break;
 	}
 
 	switch (center_channel) {