tty: serial: vt8500: fix return value check in vt8500_serial_probe()

In case of error, function of_clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/vt8500_serial.c

@@ -604,7 +604,7 @@ static int vt8500_serial_probe(struct platform_device *pdev)
 	vt8500_port->uart.flags = UPF_IOREMAP | UPF_BOOT_AUTOCONF;
 
 	vt8500_port->clk = of_clk_get(pdev->dev.of_node, 0);
-	if (vt8500_port->clk) {
+	if (!IS_ERR(vt8500_port->clk)) {
 		vt8500_port->uart.uartclk = clk_get_rate(vt8500_port->clk);
 	} else {
 		/* use the default of 24Mhz if not specified and warn */