greybus: svc: disable connection after all its users are gone

gb_svc_del() can be called during removal of gb-es2.ko module as well,
and in that case we would like to properly shutdown all modules and
interfaces as USB is still alive.

This requires that we don't disable the svc connection, at least for tx,
as that will be used while removing modules and interfaces.

Disable only rx to begin with, as we shouldn't be handling any requests
from the SVC. Disable tx only after all the users of svc connection are
gone.

Tested on EVT 2.0 by remove gb-es2.ko module. There are still few
errors, specially while quiescing the connections (-22), but not that
many.

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