m68k: Change memsz to a signed char to avoid warning

There doesn't seem to be any reason for using uchar here, so change it
to char. This fixes a warning:

pointer targets in passing argument 1 of 'sprintf' differ in signedness

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>

arch/m68k/lib/board.c

@@ -639,7 +639,7 @@ void board_init_r (gd_t *id, ulong dest_addr)
 	 */
 	{
 		ulong pram = 0;
-		uchar memsz[32];
+		char memsz[32];
 
 #ifdef CONFIG_PRAM
 		pram = getenv_ulong("pram", 10, CONFIG_PRAM);