staging: comedi: jr3_pci: factor subdevice private data init out of (*attach)

Factor the code that allocates and initializes the subdevice private data
out of the (*attach) to reduce the indent level and make the code easier
to follow.

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>