powerpc/legacy_serial: Handle SERIAL_8250_FSL=n build failures

With SERIAL_8250=y and SERIAL_8250_FSL_CONSOLE=n the both
IS_ENABLED(CONFIG_SERIAL_8250) and IS_REACHABLE(CONFIG_SERIAL_8250)
evaluate to true and so fsl8250_handle_irq() is used. However this
function is only available if CONFIG_SERIAL_8250_CONSOLE=y (and thus
SERIAL_8250_FSL=y).

To prepare SERIAL_8250_FSL becoming tristate and being enabled in more
cases, check for IS_REACHABLE(CONFIG_SERIAL_8250_FSL) before making use
of fsl8250_handle_irq(). This check is correct with and without the
change to make SERIAL_8250_FSL modular.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 66eff0ef ("powerpc/legacy_serial: Warn about 8250 devices operated without active FSL workarounds")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Message-ID: <20230609133932.786117-2-u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>