greybus: connection: Return bool from gb_connection_intf_find()

This is used only to check if an existing connection already uses the
cport_id or not and doesn't really need to return pointer to the
connection.

While at it, also rename it to suit its purpose.

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>