emu: fix copypasta where write_lo actually did write_hi

src/emu/util.rs

@@ -28,6 +28,6 @@ pub fn write_hi(reg: &Cell<u16>, val: u8) {
 pub fn write_lo(reg: &Cell<u16>, val: u8) {
   let mut buf = [0; 2];
   LittleEndian::write_u16(&mut buf, reg.get());
-  buf[1] = val;
+  buf[0] = val;
   reg.set(LittleEndian::read_u16(&buf));
 }