ixl: prevent non-privileged access to NVM update interface

Add a privilege check to the ixl_handle_nvmupd_cmd function, ensuring
that only privileged users are allowed to access the NVM update
interface.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Submitted by: Jacob Keller <jacob.e.keller@intel.com>
Reported by: markj@
Reviewed by: markj@, erj@, jeffrey.e.pieper@intel.com
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D22870