greybus: Add comment about the mode_switch_started flag

Add a comment to describe the purpose of the flag.

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