wifi: rtw89: fix null pointer access when abort scan

During cancel scan we might use vif that weren't scanning.
Fix this by using the actual scanning vif.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240119081501.25223-6-pkshih@realtek.com