serial: 8250_ce4100: Fix CONFIG_SERIAL_8250=n build

On i386, when

  CONFIG_X86_INTEL_CE=y
  # CONFIG_SERIAL_8250 is not set

it will try to compile the driver and use the stub simultaneously.
This breaks the build. Fix it by making sure that the driver
compiles only when CONFIG_SERIAL_8250 is also enabled.

On top of that ensure that CONFIG_SERIAL_8250 is actually set to 'y'
and not 'm' as the later makes no sense for this platform. The hook
may only be applied during early boot.

Fixes: acc902d ("serial: 8250: Move CE4100 quirks to a module under 8250 driver")
Fixes: 5ec6960 ("ce4100: Add errata fixes for UART on CE4100")
Reported-by: Randy Dunlap <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Signed-off-by: Andy Shevchenko <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Tested-by: Randy Dunlap <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: andy-shev

drivers/tty/serial/8250/Makefile

@@ -24,7 +24,9 @@ obj-$(CONFIG_SERIAL_8250_ASPEED_VUART)	+= 8250_aspeed_vuart.o
 obj-$(CONFIG_SERIAL_8250_BCM2835AUX)	+= 8250_bcm2835aux.o
 obj-$(CONFIG_SERIAL_8250_BCM7271)	+= 8250_bcm7271.o
 obj-$(CONFIG_SERIAL_8250_BOCA)		+= 8250_boca.o
+ifeq ($(CONFIG_SERIAL_8250),y)
 obj-$(CONFIG_X86_INTEL_CE)		+= 8250_ce4100.o
+endif
 obj-$(CONFIG_SERIAL_8250_DFL)		+= 8250_dfl.o
 obj-$(CONFIG_SERIAL_8250_DW)		+= 8250_dw.o
 obj-$(CONFIG_SERIAL_8250_EM)		+= 8250_em.o