staging: comedi: dt9812: remove 'interface' from usb private data

The usb_interface * is only used in the usb (*probe) and (*disconnect).
Remove the pointer to the 'interface' in the usb private data.

For aesthetic reasons, rename the variable 'interface' to simply
'intf' in the (*probe) and (*disconnect).

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>