greybus: kernel_ver: backport reinit_completion to pre-3.13

Backport upstream reinit_completion() to pre-3.13 kernels.

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>