tty: serial: fsl_lpuart: use kzalloc() instead of kmalloc()

Use kzalloc() instead of kmalloc() to get clean rx buffer
that is useful for DMA mode debug to check the data moving
validity.
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20190717051930.15514-4-fugang.duan@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -1097,7 +1097,7 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
 	if (sport->rx_dma_rng_buf_len < 16)
 		sport->rx_dma_rng_buf_len = 16;
 
-	ring->buf = kmalloc(sport->rx_dma_rng_buf_len, GFP_ATOMIC);
+	ring->buf = kzalloc(sport->rx_dma_rng_buf_len, GFP_ATOMIC);
 	if (!ring->buf)
 		return -ENOMEM;