uart: Fix bug in {uart8250, uart8250_mem, ns16550}_rx_byte functions

We have several different UART implementations of which three support a
timeout when receiving characters. In all of these three implementations
there is a bug where when the timeout is hit the last received character
will be returned instead of the needed 0.

The problem is that the timeout variable i is decremented after it has
been checked in the while-loop. That leads to the fact that when the
while-loop is aborted due to a timeout i will contain 0xffffffff and not
0. Thus in turn will fool the following if-statement leading to wrong
return value to the caller in this case. Therefore the caller will see a
received character event if there is none.

Change-Id: I23ff531a1e729e816764f1a071484c924dcb0f85
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/19731
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/drivers/uart/uart8250io.c

@@ -62,7 +62,8 @@ static int uart8250_can_rx_byte(unsigned base_port)
 static unsigned char uart8250_rx_byte(unsigned base_port)
 {
 	unsigned long int i = SINGLE_CHAR_TIMEOUT;
-	while (i-- && !uart8250_can_rx_byte(base_port));
+	while (i && !uart8250_can_rx_byte(base_port))
+		i--;
 
 	if (i)
 		return inb(base_port + UART8250_RBR);

src/drivers/uart/uart8250mem.c

@@ -82,8 +82,10 @@ static int uart8250_mem_can_rx_byte(void *base)
 static unsigned char uart8250_mem_rx_byte(void *base)
 {
 	unsigned long int i = SINGLE_CHAR_TIMEOUT;
-	while (i-- && !uart8250_mem_can_rx_byte(base))
+	while (i && !uart8250_mem_can_rx_byte(base)) {
 		udelay(1);
+		i--;
+	}
 	if (i)
 		return uart8250_read(base, UART8250_RBR);
 	else

src/soc/broadcom/cygnus/ns16550.c

@@ -84,8 +84,10 @@ static int ns16550_tst_byte(void)
 static unsigned char ns16550_rx_byte(void)
 {
 	unsigned long int i = SINGLE_CHAR_TIMEOUT;
-	while (i-- && !ns16550_tst_byte())
+	while (i && !ns16550_tst_byte()) {
 		udelay(1);
+		i--;
+	}
 	if (i)
 		return read32(&regs->rbr);
 	else