rtw89: remove unneeded variable

Fix the following coccicheck review:
./drivers/net/wireless/realtek/rtw89/mac.c: 1096: 5-8: Unneeded variable

Remove unneeded variable used to store return value.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211110121135.151187-1-deng.changcheng@zte.com.cn