staging: unisys: visorbus: Unregister driver on error

If there is an error in registering driver attributes, unregister
the driver as well.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>