core: make greybus_kill_gbuf not return a value

We can't do anything if killing a gbuf fails, so just make this function
"always" be successful.

At the same time, make the host controller function also be called
"kill_gbuf" to keep the terminology in sync.

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