driver/serial: ns16550: set NS16550 as default variant

This add UART_NS16550_VARIANT_16550 configuration inside the choice
of UART_NS16550_VARIANT_NS16750 and UART_NS16550_VARIANT_NS16950.
The configuration is enabled by default to make NS16550 device to get
correct FIFO size configuration (16 bytes).

fixes #45783

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>