8250_fintek - serial: 8250: Integrate Fintek into 8250_base

https://lab.nexedi.cn/kirr/linux/commit/fa01e2ca9f531b4a5693469a196eb1574b8d7d8a

The 8250_fintek driver advertises as the PNP0501 driver; however this
conflicts with the standard 16550A uart PNP0501. The conflict causes
the 8250_fintek driver to load with _every_ PNP0501, but never probe,
and causing the entire 8250 driver stack to unload if the 8250_fintek
driver is unloaded (modprobe doesn't know that 8250_pnp rather than
8250_fintek claimed the resource).

This patch merges the Fintek driver into 8250_base. On autoconfig_16550
the device is probed to verify if it is a FINTEK device or not.

This custom probing can be disabled completely via configuration. When a
Fintek device is not probed it will behave as a standard 16550A device,
with no RS485 capabilities.

Reported-by: Peter Hurley 
Signed-off-by: Ricardo Ribalda Delgado 
Reviewed-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman