Many functions obtain a 'struct qca_serdev' only to read the btsoc_type
field. Add a helper function that encapsulates this.

This also fixes crashes observed on platforms with ROME controllers
that are instantiated through ldisc and not as serdev clients. The
crashes are caused by NULL pointer dereferentiations, which stem from
the driver's assumption that a QCA HCI device is always associated with
a serdev device.

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