MLK-22340-2: ASoC: imx-pcm-rpmsg: drop the cmd I2S_TX_POINTER

MLK-22340-2: ASoC: imx-pcm-rpmsg: drop the cmd I2S_TX_POINTER

There is error log for short period size:

[ 6977.102062] i2s_rpmsg virtio0.rpmsg-audio-channel.-1.1: rpmsg_send cmd 22 timeout!

We use the I2S_TX_POINTER(cmd 22) to get the consumed period
index from M4 every period time, and m4 should send feedback
to A53. When the period time is small, there will too much
cmd send to m4, m4 may not handle it quickly that cause
timeout.

This patch is to remove the I2S_TX_POINTER cmd, m4 should
send notification to A53 every period time to be compatible
with this change. with the patch the loading of this rpmsg
channel is reduced.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
(cherry picked from commit babbc2cd8313fb5cf604b5ca93a86200df0e46b2)