staging: vchiq: delete obselete comment

This comment describes a security problem which was fixed in commit
1c954540 ("staging: vchiq: avoid mixing kernel and user pointers").
The bug is fixed now so the FIXME can be removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X/RnUjY3XkZohk7w@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>