Fix PIO count (#211)

Works fine currently for low frequency, breaks down at higher frequency e.g. 1MHz

Chased down with a scope to the 3 extra cycles needed for wait+1; mov; jmp
so account for these in demo program

pio/pio_blink/blink.c

@@ -31,5 +31,8 @@ void blink_pin_forever(PIO pio, uint sm, uint offset, uint pin, uint freq) {
     pio_sm_set_enabled(pio, sm, true);
 
     printf("Blinking pin %d at %d Hz\n", pin, freq);
-    pio->txf[sm] = clock_get_hz(clk_sys) / (2 * freq);
+
+    // PIO counter program takes 3 more cycles in total than we pass as
+    // input (wait for n + 1; mov; jmp)
+    pio->txf[sm] = (clock_get_hz(clk_sys) / (2 * freq)) - 3;
 }