staging: comedi: usbduxfast: rename 'urbIn' in private data

Rename the CamelCase variable 'urbIn' in the private data to
simply 'urb'.

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>