rtl8xxxu: Remove set but not used variable 'rsr'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c: In function rtl8xxxu_gen2_config_channel:
drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:1266:13: warning: variable rsr set but not used [-Wunused-but-set-variable]

rsr is never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>