staging: unisys: visorbus: don't unregister if we failed to register

If we fail to register the visordriver, don't call unregister, just
return with the error.

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>