ASoC: SOF: Intel: hda: remove unnecessary ROM IPC filter function

The HDA_DSP_IPC_PURGE_FW IPC from ROM is already handled in
cl_dsp_init(), and as IPC IRQ is disabled at this stage, this
IPC will be never received in the IRQ thread. The function
hda_dsp_ipc_is_sof for filtering the ROM IPC can be removed
safely.

Signed-off-by: Amery Song <chao.song@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Keyon <yang.jie@intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20200312200622.24477-5-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>