serial: 8250_exar: No need to autoconfigure Exar ports

Since we have a separate driver there is no need to autoconfigure ports,
we already know what they are.

Drop autoconfiguration in 8250_port and move type detection to 8250_exar.

Cc: Aaron Sierra <asierra@xes-inc.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190731170558.52897-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>