Bluetooth: hci_qca: Add wrapper functions for setting UART speed

In function qca_setup, we set initial and operating speeds for Qualcomm
Bluetooth SoC's. This block of code is common across different
Qualcomm Bluetooth SoC's. Instead of duplicating the code, created
a wrapper function to set the speeds. So that future coming SoC's
can use these wrapper functions to set speeds.

Signed-off-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>