serial: Indicate fintek option may also be required for RS232 support

The current config comment for SERIAL_8250_FINTEK implies that this
option is only needed when one wants to support RS485. As it turns
out we also need to enable this option for RS232 support to function
correctly on some variants.

For example for variants such as the F71869AD attempting to use
multiple RS232 ports simultaneously without this option enabled can
result in data corruption.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Link: https://lore.kernel.org/r/20230521075046.3539376-1-james.hilliard1@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>