wifi: rtw89: pci: correct TX resource checking in low power mode

Number of TX resource must be minimum of TX_BD and TX_WD. Only considering
TX_BD could drop TX packets pulled from mac80211 if TX_WD is unavailable.

Fixes: 52edbb9f

 ("rtw89: ps: access TX/RX rings via another registers in low power mode")
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220824063312.15784-2-pkshih@realtek.com