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

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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>