serial: pl011: Avoid assumptions about buffer size when refilling TX DMA

The existing code assumed that PL011_DMA_BUFFER_SIZE == UART_XMIT_SIZE,
which may not always be the case.  This allows for these two being
different sizes and not copying too much data.
Signed-off-by: NAndrew Jackson <Andrew.Jackson@arm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -501,7 +501,11 @@ static int pl011_dma_tx_refill(struct uart_amba_port *uap)
 		memcpy(&dmatx->buf[0], &xmit->buf[xmit->tail], count);
 	else {
 		size_t first = UART_XMIT_SIZE - xmit->tail;
-		size_t second = xmit->head;
+		size_t second;
+
+		if (first > count)
+			first = count;
+		second = count - first;
 
 		memcpy(&dmatx->buf[0], &xmit->buf[xmit->tail], first);
 		if (second)