amic: don't reset cur_buf_pos if we've drained the DMA buffer

Otherwise if pull_data is called again, it will think that it still
has data available in the buffer (rem_len will be non-zero) and
random data at the buffer location will be returned.

This manifested itself as noise being played back in the JS
implementation of the SoundServer. The cubeb implementation was not
affected because it stops polling once it's told it has no more
data in the buffer. Both approaches are valid (the JS version pads
data with silence), and the DMA buffer should support both.

devices/ioctrl/amic.cpp

@@ -546,7 +546,6 @@ DmaPullResult AmicSndOutDma::pull_data(uint32_t req_len, uint32_t *avail_len,
         // if it's false stop delivering sound data
         // this will effectively stop audio playback
         if (this->snd_buf_num && !this->enabled) {
-            this->cur_buf_pos = 0;
             return DmaPullResult::NoMoreData;
         }