serial: 8250: allow disabling of Freescale 16550 compile test

serial: 8250: allow disabling of Freescale 16550 compile test

The SERIAL_8250_FSL option is used to enable a workaround for a
break-detection erratum for Freescale 16550 UARTs in the 8250 driver and
is currently also used to enable support for ACPI enumeration.

It is enabled on PPC, ARM and ARM64 whenever 8250 console support is
enabled (since the quirk is needed for sysrq handling).

Commit 8a0b4c087aef ("serial: 8250: extend compile-test coverage")
enabled compile testing of the code in question but did not provide a
means to disable the option when COMPILE_TEST is enabled.

Add a conditional input prompt instead so that SERIAL_8250_FSL is no
longer enabled by default when compile testing while continuing to
always enable the quirk for platforms that may need it.

Fixes: 8a0b4c087aef ("serial: 8250: extend compile-test coverage")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210924141232.4419-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>