Bluetooth: hci_ll: set operational frequency earlier

Uploading the firmware needs quite a few seconds if done at 115200 kbps. So set
the operational frequency, usually 3 MHz, before uploading the firmware.

I have successfully tested this with a wl1837mod.

Signed-off-by: Philipp Puschmann <philipp.puschmann@emlix.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>