m68k: Disable Amiga serial console support if modular

If CONFIG_AMIGA_BUILTIN_SERIAL=m, I get the following warnings:

| drivers/char/amiserial.c: At top level:
| drivers/char/amiserial.c:2138: warning: data definition has no type or storage class
| drivers/char/amiserial.c:2138: warning: type defaults to 'int' in declaration of 'console_initcall'
| drivers/char/amiserial.c:2138: warning: parameter names (without types) in function declaration
| drivers/char/amiserial.c:2134: warning: 'amiserial_console_init' defined but not used

because console_initcall() is not defined (nor really sensible) in the
modular case.

So disable serial console support if the driver is modular.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>