greybus: connection: export gb_connection_disable_rx() fn

gb_connection_disable_rx() fn is required to be used by other modules
(e.g. bridged-phy drivers) and so export it.

Testing Done: Tested on gbsim.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>