ASoC: SOF: Do not send cmd via SHIM register

We use shim registers only to notify the other
side that a message was sent. The actual information
for the message is transmitted via mailbox.

cmd information inside shim register is not used by
the DSP, so we remove it to avoid confusion.

Signed-off-by: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>