staging: comedi: rtd520: cleanup pci bar ioremap

Use the size returned by pci_resource_len() when ioremap'ing the
pci bars instead of assuming a size.

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>