staging: unisys: visorbus: don't ignore visorchannel_read error

Don't override the visorchannel_read_error, just return the
error that it generates.

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>