m68k: amiga: Clean up Amiga hardware configuration

Move the generic Amiga hardware configuration section out of the
switch statement, which allows to replace all ugly jumps by break
statements.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200826125124.23863-1-geert@linux-m68k.org