firmware: imx: scu: Ensure sequential TX

SCU requires that all messages words are written sequentially but linux MU
driver implements multiple independent channels for each register so ordering
between different channels must be ensured by SCU API interface.

Wait for tx_done before every send to ensure that no queueing happens at the
mailbox channel level.

Fixes: edbee095

 ("firmware: imx: add SCU firmware driver support")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: : Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>