wifi: rtw89: fix null vif pointer when get management frame date rate

When transmitting a packet that gets from ieee80211_nullfunc_get(),
the vif in tx_info->control was no assigned, which will cause
dereferencing a null pointer.

Signed-off-by: Kuan-Chung Chen <damon.chen@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230106121517.19841-1-pkshih@realtek.com