staging: comedi: fix cast warning in comedi_compat32.c

This patch fixes the following sparse warning:

drivers/staging/comedi/comedi_compat32.c:205:16: warning: cast removes
address space of expression

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>