staging: comedi: jr3_pci: tidy up subdevice init

For aesthetics, use a pointer to the comedi_subdevice instead of
accessing the dev->subdevices array directly.

Move the local variable for the subdevice private data so that
this function does not declare the variable twice.

Change the kzalloc for the subdevice private data to remove the
sizeof(struct foo).

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>