greybus: svc: No need to return errors from [gb_]svc_connection_destroy()

These routines are responsible to destroy a connection that is going
away, the return value is of no use. At best, print an error message to
show that we got an error.

Make their return type void.

Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>