greybus: connection: drop the connection_mutex

Drop the useless connection_mutex that did not, and can not be used to
prevent connection lookup races in atomic context and therefore serves
no purpose.

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>