serial: mxs-auart: Index is unsigned

Fix the following warning when building with W=1 option:

drivers/tty/serial/mxs-auart.c: In function 'mxs_auart_tx_chars':
drivers/tty/serial/mxs-auart.c:272:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mxs-auart.c

@@ -253,7 +253,7 @@ static void mxs_auart_tx_chars(struct mxs_auart_port *s)
 	struct circ_buf *xmit = &s->port.state->xmit;
 
 	if (auart_dma_enabled(s)) {
-		int i = 0;
+		u32 i = 0;
 		int size;
 		void *buffer = s->tx_dma_buf;