wifi: rtl8xxxu: check vif before using in rtl8xxxu_tx()

The 'vif' is from tx_info of SKB, and other codes check 'vif' before using,
which raises smatch warnings:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:5656 rtl8xxxu_tx()
   warn: variable dereferenced before check 'vif' (see line 5553)

Compile tested only.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240216033923.34683-1-pkshih@realtek.com