wifi: rtl8xxxu: Fix reading the vendor of combo chips

The wifi + bluetooth combo chips (RTL8723AU and RTL8723BU) read the
chip vendor from the wrong register because the val32 variable gets
overwritten. Add one more variable to avoid this.

This had no real effect on RTL8723BU. It may have had an effect on
RTL8723AU.

Fixes: 26f1fad2 ("New driver: rtl8xxxu (mac80211)")
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/24af8024-2f07-552b-93d8-38823d8e3cb0@gmail.com