staging: comedi: dt9812: remove unused variables from private data

The vendor, product, and serial numbers read from the usb device
are only used for a dev_info() message about the device after it
is reset. Reading these values might not be required for the usb
device to function.

For now just remove the variables from the private data and just
use local variables.

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>