staging: comedi: jr3_pci: fix iomem dereference

Correct a direct dereference of I/O memory to use an appropriate I/O
memory access function.  Note that the pointer being dereferenced is not
currently tagged with `__iomem` but I plan to correct that for 3.7.

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