staging: unisys: visorbus: return EAGAIN when empty

It really isn't an error when you call visorchannel_signalremove and
there isn't anything in the queue. Just means that the IOSP didn't
process anything since the last time you checked. Just inform the
caller that it is empty by returning -EAGAIN.

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