staging: comedi: drivers: replace SDF_WRITEABLE with SDF_WRITABLE

As indicated in the comedi.h uapi header, SDF_WRITEABLE was a spelling
error in the API, SDF_WRITABLE is prefered.

For aesthetics, replace all the SDF_WRITEABLE uses with SDF_WRITABLE.

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>