staging: comedi: remove superfluous retval = 0 in comedi_read()

`comedi_read()` initializes `retval` to 0.  The other `retval = 0`
assignments are superfluous, so remove them.

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>