Bluetooth: btusb: check for NULL in btusb_find_altsetting()

The new btusb_find_altsetting() dereferences it without checking
the check is added in this patch

Signed-off-by: Sathish Narasimman <sathish.narasimman@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>