greybus: connection: make connection enable/disable thread safe

Add connection mutex to protect connection enabling and disabling.

This is needed to eventually allow drivers to manage the state of their
connections during operation (i.e. post probe and pre disconnect).

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