staging: unisys: visorbus change -1 return values

This patch changes the vague -1 return values to -EFAULT since
it would be the most appropriate, given that this error
would only occur in an unexpected bad offset field.
Resulting in a bad address.

Signed-off-by: Erik Arfvidson <erik.arfvidson@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>