staging: unisys: visorbus: add error handling around kobject_uevent

The function kobject_uevent_env can return an error, pass that error
to the caller so it can be handled correctly.

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>