greybus: get rid of a compile warning

The compiler has no way of knowing whether a called function will
actually assign something to the object whose address is passed as
an argument.  So it must assume it won't happen, and this leads to a
compile warning.  Fix this.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>