tty: serial: Add missing module license for 8250_base.ko

The split of the 8250 driver into a 8250_base/8250.ko resulted in a
lack of a license for the 8250_base.ko module. This caused the module
to fail to load and the kernel to be tainted. Add the appropriate
MODULE_LICENSE to 8250_port.c, which is always compiled into
8250_base.ko

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reported-by: Mikael Pettersson <mikpelinux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -2910,3 +2910,5 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
 }
 
 #endif /* CONFIG_SERIAL_8250_CONSOLE */
+
+MODULE_LICENSE("GPL");