mt76: fix possible NULL pointer dereferencing in mt76x2_mac_write_txwi()

Verify wcid is not NULL before dereferencing the pointer to initialize
txwi rate/power info

Fixes: 7bc04215 ("mt76: add driver code for MT76x2e")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>