staging: comedi: ni_670x: remove spurious free_irq() call

This driver's comedi `detach` handler (`ni_670x_detach()`) calls
`free_irq()` but the driver doesn't call `request_irq()` anywhere.
Remove the spurious call.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>