staging: comedi: jr3_pci: remove unneeded 'spriv' checks

If `jr3_pci_auto_attach()` returns with no error, we can now be sure
that the COMEDI subdevice private data structures have been allocated.
Remove the tests for a valid pointer to the private data structure in
`jr3_pci_ai_insn_read()`, `jr3_pci_open()`, and
`jr3_pci_poll_subdevice()`, since they will not be called if the pointer
is invalid.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>