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>