staging: comedi: ni_6527: tidy up ni6527_intr_cmd()

For aesthetics, use a local variable for the __iomem base address
used to write to the registers.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>