ns16550: do not override fifo size if explicitly set

ns16550: do not override fifo size if explicitly set

If fifo size is already set via uart_params, do not force it to 16 - which
may not match the actual hardware. Specifically Exar cards have fifo of
256 bytes.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>