x1000: default to sending 0 samples on underflow

Previously it used the last sample, which turns out to be broken.
The AIC appears to send out a random non-zero sample continously
after boot -- probably because the last sample is uninitialized by
the hardware. Disabling playback is supposed to make it send zero
samples irregardless of the LSMP bit according to the docs, but it
doesn't seem to work this way.

- Change eliminates the white noise heard on the M3K after boot.
- Change has no adverse effects on the Q1.
- Leave Eros Q on the old setting since sending zeros exacerbates
  clicking due to DAC's automute feature.

Change-Id: I9996793fc34f4475b19700d076b11505353b3836

firmware/target/mips/ingenic_x1000/erosqnative/audiohw-erosqnative.c

@@ -42,6 +42,7 @@ void audiohw_init(void)
     gpio_set_level(GPIO_ISL54405_MUTE, 1);
     gpio_set_level(GPIO_PCM5102A_XMIT, 0);
 
+    aic_set_play_last_sample(true);
     aic_set_external_codec(true);
     aic_set_i2s_mode(AIC_I2S_MASTER_MODE);
     audiohw_set_frequency(HW_FREQ_48);

firmware/target/mips/ingenic_x1000/pcm-x1000.c

@@ -58,7 +58,7 @@ void pcm_play_dma_init(void)
     /* Configure AIC with some sane defaults */
     jz_writef(AIC_CFG, RST(1));
     jz_writef(AIC_I2SCR, STPBK(1));
-    jz_writef(AIC_CFG, MSB(0), LSMP(1), ICDC(0), AUSEL(1), BCKD(0), SYNCD(0));
+    jz_writef(AIC_CFG, MSB(0), LSMP(0), ICDC(0), AUSEL(1), BCKD(0), SYNCD(0));
     jz_writef(AIC_CCR, ENDSW(0), ASVTSU(0));
     jz_writef(AIC_I2SCR, RFIRST(0), ESCLK(0), AMSL(0));
     jz_write(AIC_SPENA, 0);