staging: comedi: comedi_usb: allow comedi usb drivers to pass a 'context'

Allow the comedi usb drivers to pass a 'context' from their (*probe)
functions to the comedi core's comedi_usb_auto_config(). This 'context'
is then passed to comedi_auto_config() and then to the comedi_driver's
(*auto_attach).

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>