serial: omap-serial: fix signess error

"ret" is unsigned, so check for (ret < 0) made no sense.
Made it signed.
Signed-off-by: NVasiliy Kulikov <segooon@gmail.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com
Signed-off-by: NTony Lindgren <tony@atomide.com>

drivers/serial/omap-serial.c

@@ -1055,7 +1055,7 @@ static void serial_omap_rx_timeout(unsigned long uart_no)
 {
 	struct uart_omap_port *up = ui[uart_no];
 	unsigned int curr_dma_pos, curr_transmitted_size;
-	unsigned int ret = 0;
+	int ret = 0;
 
 	curr_dma_pos = omap_get_dma_dst_pos(up->uart_dma.rx_dma_channel);
 	if ((curr_dma_pos == up->uart_dma.prev_rx_dma_pos) ||