serial: mpc52xx_uart: fix null pointer dereference

Commit 6acc683351
introduced NULL pointer dereference and kernel crash
on ppc32 machines while booting. Fix this bug now.

Reported-by: Leonardo Chiquitto <leonardo.lists@gmail.com>
Tested-by: Leonardo Chiquitto <leonardo.lists@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/serial/mpc52xx_uart.c

@@ -1467,7 +1467,7 @@ mpc52xx_uart_init(void)
 	/*
 	 * Map the PSC FIFO Controller and init if on MPC512x.
 	 */
-	if (psc_ops->fifoc_init) {
+	if (psc_ops && psc_ops->fifoc_init) {
 		ret = psc_ops->fifoc_init();
 		if (ret)
 			return ret;