staging: comedi: pcmda12: tidy up zero_chans()

Rename the function so it has namespace associated with the driver.

For aesthetic reasons, move the function closer to it's only caller.

Pass the comedi_subdevice pointer to the function so we can get the
number of channels to reset from it instead of using the 'CHANS'
define.

Remove the 'CHANS' define since it's a very generic name.

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>