staging: unisys: visornic - check visorchannel_signalinsert/remove failures

Logic to check for failures of visorchannel_signalinsert() and
visorchannel_signalremove() were added, and a new sent_post_failed counter
tracks the number of times we failed to post a rcv buffer to the IO
partition.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>