powerpc/legacy_serial: Warn about 8250 devices operated without active FSL workarounds

If the 8250 driver is built as a module (or built-in without console
support) the Freescale specific workaround were silently not activated.
Add a warning in this case.

Currently CONFIG_SERIAL_8250_FSL=y implies that the function
fsl8250_handle_irq() is built-in and can be used. However with the
changes of the next commit CONFIG_SERIAL_8250_FSL might be enabled also
when the 8250 driver is a module and so more care is needed when
fsl8250_handle_irq() is to be used. The code added here is able to
handle the new situation already.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Message-ID: <20230605130857.85543-2-u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>