staging: comedi: fix sparse warning in do_devconfig_ioctl()

For the COMEDI_DEVCONFIG ioctl, the user application may embed a pointer
to firmware data within a designated element (or two elements for 64-bit
pointers) of the `options[]` array in the `struct comedi_devconfig`.
`do_devconfig_ioctl()` calls `comedi_aux_data()` to extract the pointer
value. It needs to be treated as a `__user` pointer so the firmware
data can be copied into kernel memory, so cast the result of
`comedi_aux_data()` to avoid a "sparse" warning. This is not ideal but
`comedi_aux_data()` is called elsewhere in a wholly kernel memory
context so we can't just change its return type to include the `__user`
tag.

Signed-off-by: Ian Abbott <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/comedi/comedi_fops.c

···
 			return -ENOMEM;
 
 		if (copy_from_user(aux_data,
-				   comedi_aux_data(it.options, 0), aux_len)) {
+				   (unsigned char __user *
+				    )comedi_aux_data(it.options, 0), aux_len)) {
 			vfree(aux_data);
 			return -EFAULT;
 		}