serial: ns16550: Correct the base address type

Currently ns16550_serial_assign_base() treats the argument 'base'
with type `ulong`. This is incorrect because the base address was
obtained from device tree with type `fdt_addr_t` that can represent
a physical address larger than 32-bit in a 32-bit system.

Fixes: 9e6ce621 ("serial: ns16550: Fix ordering of getting base address")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>