staging: comedi: c6xdigio: add readback of last pwm channel values

Add and (*insn_read) for the PWM subdevice to allow reading back the
last value written to the channels.

There are only 2 PWM channels and they have a maxdata of 500. Pack
the last values in the subdevice 'state' instead of adding a private
data struct to this driver.

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>