ath6kl: fix missing uart debug pin for 6004 HW 3.0

For some reason, the 6004 HW 3.0 definition was missing the value for the
uarttx_pin (used for firmware debug). This corrects this situation.

Signed-off-by: Steve deRosier <steve.derosier@lairdtech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>