staging: comedi: usbdux: remove usbdux_pwm_read()

The pwm subdevice is not readable so this callback function is not
needed. Remove it.

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