High PC speaker frequencies treated as constant level.

Improved timing for port 0x61 writes.

src/keyboard_at.c

@@ -284,6 +284,10 @@ void keyboard_at_write(uint16_t port, uint8_t val, void *priv)
                
                 case 0x61:
                 ppi.pb = val;
+
+                timer_process();
+                timer_update_outstanding();
+
                 speaker_gated = val & 1;
                 speaker_enable = val & 2;
                 if (speaker_enable)