staging: comedi: s626: rename private data 'base_addr' variable

The base_address variable in the private data is the ioremap'ed
PCI bar 0 resource.

For aesthetic reasons, and to shorten some of the lines, rename
this variable to 'mmio'.
Signed-off-by: NH Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: NIan Abbott <abbotti@mev.co.uk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>