staging: comedi: use memdup_user to simplify code

Use memdup_user rather than duplicating implementation. Fix following
coccinelle warnings:

drivers/staging/comedi/comedi_fops.c:1425:5-12: WARNING opportunity for memdup_user
drivers/staging/comedi/comedi_fops.c:1553:6-13: WARNING opportunity for memdup_user

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>