Input: at32psif - do not sleep in atomic context

We can't use msleep() while holding a spinlock, moreower serio's write()
method is supposed to be useable from inettrupt context. Let's do what
i8042 does and poll the status register every 50 us (with udelay).
Reported-by: NMarjan Fojkar <marjan@pajkc.eu>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

drivers/input/serio/at32psif.c

@@ -137,7 +137,7 @@ static int psif_write(struct serio *io, unsigned char val)
 	spin_lock_irqsave(&psif->lock, flags);
 
 	while (!(psif_readl(psif, SR) & PSIF_BIT(TXEMPTY)) && timeout--)
-		msleep(10);
+		udelay(50);
 
 	if (timeout >= 0) {
 		psif_writel(psif, THR, val);