ASoC: SOF: Remove call to snd_sof_dsp_mailbox_init

ASoC: SOF: Remove call to snd_sof_dsp_mailbox_init

This is reserved for some historical reason, we didn't enable memory
windows for byt/bdw at the beginning, to make it compatible, we get
those mailbox offsets from fw_ready struct firstly, and then update them
if they existed in the following memory windows, to make sure the
mailbox still can be used if no memory windows are created.

With this change all platforms have the same implementation for
xxx_fw_ready function so that we can refactor it in a common file.

Suggested-by: Keyon Jie <yang.jie@linux.intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190807150203.26359-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>