m68k/atari: Avoid VLA use in atari_switches_setup()

With gcc 7.3.0:

    arch/m68k/atari/config.c: In function ‘atari_switches_setup’:
    arch/m68k/atari/config.c:151:2: warning: ISO C90 forbids variable length array ‘switches’ [-Wvla]
      char switches[strlen(str) + 1];
      ^~~~

Replace the variable size by the maximum kernel command line size (256
bytes), which is an upper limit for all suboptions.

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