slimbus: messaging: pass correct wbuf

There seems to be a typo while filling msg for slim_write, wbuf is
set to NULL instead of rbuf.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>