staging: comedi: ke_counter: rename 'subdevice' variable to 's'

Rename the variable used for the comedi_subdevice pointer from
'subdevice' to 's'. This is more typical in other comedi drivers
and helps when searching with grep.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>