ath10k: set the UART baud rate to 19200

When configuring the host_interests over BMI, set the UART
baud rate to 19200. This is valid for QCA988X_2.0 devices.

kvalo: found during code review, there should not be any functionality
changes

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>