Bluetooth: hci_qca: Fix crash with non-serdev devices

qca_set_baudrate() calls serdev_device_wait_until_sent() assuming that
the HCI is always associated with a serdev device. This isn't true for
ROME controllers instantiated through ldisc, where the call causes a
crash due to a NULL pointer dereferentiation. Only call the function
when we have a serdev device. The timeout for ROME devices at the end
of qca_set_baudrate() is long enough to be reasonably sure that the
command was sent.

Fixes: fa9ad876 ("Bluetooth: hci_qca: Add support for Qualcomm Bluetooth chip wcn3990")
Reported-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Reported-by: Rocky Liao <rjliao@codeaurora.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Rocky Liao <rjliao@codeaurora.org>
Tested-by: Rocky Liao <rjliao@codeaurora.org>
Reviewed-by: Balakrishna Godavarthi <bgodavar@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>