staging: comedi: pcl816: tidy up pcl818_check()

This function probes a number of the boards registers during the
(*attach) to verify that it is actually a PCL-816 compatible board.
For aesthetics, move the function closer to the (*attach).

To better match the pcl818 driver, allocate the private data before
calling pcl816_check().

Refactor the function to return an errno if fails. Change the errno
from -EIO to -ENODEV and remove the unnecessary dev_err() noise.

Make sure the CONTROL register is reset to a known state after the
check. The 0x18 value actually defines an invalid interrupt selection
and sets an undefined bit.

Add a couple comments to clarify the magic values.

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>