serial: 8250_omap: fix kernel crash in suspend-to-ram

omap_device infrastructure has a suspend_noirq hook which
runtime suspends all devices late in the suspend cycle (see
_od_suspend_noirq() in arch/arm/mach-omap2/omap_device.c)

This leads to a NULL pointer exception in 8250_omap driver
since by the time omap8250_runtime_suspend() is called, 8250_dma
driver has already set rxchan to NULL via serial8250_release_dma().

Make an explicit check to see if rxchan is NULL in
runtime_{suspend|resume} hooks to fix this.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -1285,7 +1285,7 @@ static int omap8250_runtime_suspend(struct device *dev)
 			return -EBUSY;
 	}
 
-	if (up->dma)
+	if (up->dma && up->dma->rxchan)
 		omap_8250_rx_dma(up, UART_IIR_RX_TIMEOUT);
 
 	priv->latency = PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE;
@@ -1310,7 +1310,7 @@ static int omap8250_runtime_resume(struct device *dev)
 	if (loss_cntx)
 		omap8250_restore_regs(up);
 
-	if (up->dma)
+	if (up->dma && up->dma->rxchan)
 		omap_8250_rx_dma(up, 0);
 
 	priv->latency = priv->calc_latency;