staging: sep: Fix sign of error

One of our errors wasn't negative as intended. Fix this.
(Found by Hillf Danton)

While we are at it turn user causable messages down to dev_dbg level in the
ioctl paths.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>