staging: comedi: ni_labpc_common: move comedi_alloc_devpriv() to common code

The ni_labpc_common module is used by the ni_labpc, ni_labpc_pci, and ni_labpc_cs
drivers to provide the common code support. Those drivers currently all call
comedi_alloc_devpriv() to allocate the private data before calling the common
attach function.

For aesthetics, move the private data allocation into the common code.

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>