tty: serial: altera_uart: Check for NULL platform_data in probe.

Follow altera_jtag_uart.  This fixes a crash if there is a mistake in the DTS.

Signed-off-by: Yuriy Kozlov <ykozlov@ptcusa.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>