amba_pl011: Insert mb() for correct FIFO handling

The pl011 register accessor functions use the _relaxed versions of the
standard readl() and writel() functions, meaning that there are no
automatic memory barriers. When polling a FIFO status register to check
for fullness, it is necessary to ensure that any outstanding writes have
completed; otherwise the flags are effectively stale, making it possible
that the next write is to a full FIFO.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>

drivers/tty/serial/amba-pl011.c

@@ -1495,6 +1495,7 @@ static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c,
 		return false; /* unable to transmit character */
 
 	pl011_write(c, uap, REG_DR);
+	mb();
 	uap->port.icount.tx++;
 
 	return true;