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 <[e-mail]>

arch/m68k/atari/config.c

···
 /* 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;