Reset ellasped time when start playing a new song...

When playing multiple files, ellasped time wasn’t reset to 0.
In order to fix that, and according to datasheet (chapter 9.6.5 "SCI_DECODE_TIME (RW)"), it seems that we need to write twice a 0 in REG_DECODETIME.

Tested on an Arduino Micro + VS1053 micro shield (https://github.com/paulgreg/arduino_mp3_player).