mei: rename to mei_host_buffer_is_empty to hbuf_is_ready

we rename the mei_host_buffer_is_empty to keep naming
convention of hbuf and also make the query more generic
to be correct also for other under laying hardware

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>