staging: comedi: usbdux: remove an unnecessary dev_info()

usbdux_attach_common() prints two messages via dev_info() that shows a
device has been attached.  The first of these messages includes an index
into a static array that the function determines by pointer subtraction,
assuming the pointer passed to the function points to an element of the
array.  Dan Carpenter pointed out that this was kind of ugly.  Since the
dev_info() that prints the array index doesn't add anything useful
(since no other messages print the array index and nothing else uses
it), let's just get rid of it.

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