Revert "removed faulty JiffyDOS optimization (compatibility problems)"

This reverts commit 02863c5cf5.

c64/kernal/iec/unlsn.s

@@ -18,6 +18,13 @@ UNLSN:
 	// According to serial-bus.pdf (page 15) this routine flushes the IEC out buffer
 	jsr iec_tx_flush
 
+#if CONFIG_IEC_JIFFYDOS
+
+	lda #$00 // switch back to standard protocol
+	sta IECPROTO
+
+#endif
+
 	// Buffer empty, send the command
 	lda #$3F