staging: unisys: visorbus: cleanup error handling in visorbus_uevent

The add_uevent_var returns an error. Don't overwrite the error with
-ENOMEM, just pass the error code back up.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>