staging: comedi: vmk80xx: remove usb_interface from private data

This driver uses the comedi auto attach mechanism so the comedi_device
will always have an associated usb_interface and usb_device.

Remove the unnecessary checks if the comedi_device has a usb_interface
attached. This also allows removing the usb_interface from the private
data.

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>