staging: comedi: comedi_fops: cleanup do_devinfo_ioctl()

Consolidate the local variables 'read_subdev' and 'write_subdev' into a
single local variable 's'.

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>