serial/8250_fintek: Support keys different than default

Chip can be configured to use entry key different than 0x77. Try all the
valid keys until one gives out the right chip id.

Reported-by: Peter Hong <peter_hong@fintek.com.tw>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>