staging: comedi: c6xdigio: remove unnecessary c6xdigio_pwmo_insn_read()

The pwm subdevice in this driver sets the subdev_flags to SDF_WRITEABLE
so the s->insn_read callback is not needed. Since this callback simply
outputs a printk(KERN_DEBUG ... message jsut 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>