staging: rtl8723bs: core: rtw_recv: Mark debug variable as __maybe_unused

Fixes the following W=1 kernel build warning(s):

 from drivers/staging/rtl8188eu/core/rtw_recv.c:12:
 drivers/staging/rtl8723bs/core/rtw_recv.c: In function ‘rtw_signal_stat_timer_hdl’:
 drivers/staging/rtl8723bs/core/rtw_recv.c:2341:6: warning: variable ‘num_signal_qual’ set but not used [-Wunused-but-set-variable]

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210414181129.1628598-14-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/core/rtw_recv.c

@@ -2123,7 +2123,7 @@ static void rtw_signal_stat_timer_hdl(struct timer_list *t)
 	u8 avg_signal_strength = 0;
 	u8 avg_signal_qual = 0;
 	u32 num_signal_strength = 0;
-	u32 num_signal_qual = 0;
+	u32 __maybe_unused num_signal_qual = 0;
 	u8 _alpha = 5; /*  this value is based on converging_constant = 5000 and sampling_interval = 1000 */
 
 	if (adapter->recvpriv.is_signal_dbg) {