staging: comedi: don't call attach_usb handler

All the Comedi drivers that call `comedi_usb_auto_config()` have
replaced the `attach_usb()` handler in their `struct comedi_driver` with
a `auto_attach()` handler, so there is no need to check for the
existence of the `attach_usb()` handler any more.  Remove this check and
the code that calls it.

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