staging: unisys: visornic: add error handling for visorchannel_signalinsert/remove

Since signalinsert/remove now return valid error codes, we need to check
them when we call them in visornic. The error codes need to propagate out
to the calling functions.

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>