m68k: amiga: config: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: allmodconfig m68k):

arch/m68k/amiga/config.c: In function ‘amiga_identify’:
./arch/m68k/include/asm/amigahw.h:42:50: warning: this statement may fall through [-Wimplicit-fallthrough=]
 #define AMIGAHW_SET(name) (amiga_hw_present.name = 1)
                           ~~~~~~~~~~~~~~~~~~~~~~~^~~~
arch/m68k/amiga/config.c:223:3: note: in expansion of macro ‘AMIGAHW_SET’
   AMIGAHW_SET(PCMCIA);
   ^~~~~~~~~~~
arch/m68k/amiga/config.c:224:2: note: here
  case AMI_500:
  ^~~~

Replace the existing /* fall through */ comments and fix the issue above
by using the new pseudo-keyword fallthrough;

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/14ff577604d25243c8a897f851b436ba87ae87cb.1585264062.git.gustavo@embeddedor.com


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>