Fix Clear Screen implementation of HD44780

Datasheet mandates filling DDRAM with spaces, setting address
counter to DDRAM address 0, and setting entry mode I/D to 1
(increment mode).

The "filling with spaces" was implemented but the rest was missing.
Now added.

examples/parts/hd44780.c

@@ -56,6 +56,8 @@ _hd44780_clear_screen(
 		hd44780_t *b)
 {
 	memset(b->vram, ' ', 0x80);
+	b->cursor = 0;
+	hd44780_set_flag(b, HD44780_FLAG_I_D, 2);
 	hd44780_set_flag(b, HD44780_FLAG_DIRTY, 1);
 	avr_raise_irq(b->irq + IRQ_HD44780_ADDR, b->cursor);
 }