staging: comedi: remove memset() call from comedi_device_init()

`comedi_device_init()` is only called from one place
(`comedi_alloc_board_minor()`) and the `struct comedi_device` has
already been zeroed out by `kzalloc()`.  Don't bother zeroing it out
again with `memset()`.

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