ath6kl: add ability to set debug uart baud rate

It's useful to permit the customization of the debug uart baud rate. Enable
this and send down the value to the chip if we're enabling debug.

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