staging: vchiq_arm: Service callbacks must not fail

Service callbacks are not allowed to return an error. The internal
callback that delivers events and messages to user tasks does not
enqueue them if the service is closing, but this is not an error
and should not be reported as such.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>