greybus: camera: Initialize mutex before using it

We are using the mutex from gb_camera_cleanup(), which can get called
even before the mutex is initialized.

Fix it by initializing the mutex early enough.

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