greybus: core: rename greybus_deregister()

Rename greybus_deregister() to be greybus_deregister_driver(), so
its name mirrors the greybus_register_driver() function it matches.
Define greybus_deregister() to be a trivial macro.

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