staging: comedi: adl_pci9118: allocate private data in pci9118_common_attach()

The (*auto_attach) and legacy (*attach) both allocate the private data but
don't use it. For aesthetics, move the allocation into the common attach
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>