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>

arch/m68k/atari/config.c

@@ -148,7 +148,7 @@ int __init atari_parse_bootinfo(const struct bi_record *record)
 /* Parse the Atari-specific switches= option. */
 static int __init atari_switches_setup(char *str)
 {
-	char switches[strlen(str) + 1];
+	char switches[COMMAND_LINE_SIZE];
 	char *p;
 	int ovsc_shift;
 	char *args = switches;