staging: comedi: support auto_attach() for PCI and USB

Allow `comedi_pci_auto_config()` and `comedi_usb_auto_config()` to use
the new `auto_attach()` hook in the low-level driver's `struct
comedi_driver` if it is set and the `attach_pci()` or `attach_usb()`
hook (for PCI or USB respectively) is `NULL`.

Eventually, the `auto_attach()` hook will be the only way of
auto-configuring hardware devices as comedi devices and the bus-type
specific `attach_pci()` and `attach_usb()` hooks will be removed.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>