staging: comedi: ni_mio_common: tidy up the PFI subdevice init

For aesthetics, add some whitespace to the subdevice init and
tidy it up a bit.

Remove the need for the extra local variable.

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>