greybus: ignore a null cookie when canceling buffer

It's possible for an in-flight buffer to be recorded as sent *after*
a thread has begin the process of canceling it.  In that case the
Greybus message cookie will be set to NULL, and that value can end
up getting passed to buffer_cancel().  Change buffer_cancel() so
it properly handles (ignores) a null cookie pointer.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>