greybus: connection: prevent oops for protocol_id sysfs file

If we don't have a protocol assigned to a connection, don't oops when
trying to read the "protocol_id" sysfs file.

Fixes Jira SW-968.

Reported-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>