staging: comedi: jr3_pci: fix iomem dereference

commit e1878957b4676a17cf398f7f5723b365e9a2ca48 upstream.

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.

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