staging: mt7621-mmc: fix compile warnging: cmd_buf

cmd_buf is only used when MT6575_SD_DEBUG is defined.
So only declare it in that case.
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-mmc/dbg.c

@@ -51,7 +51,6 @@
 #include "mt6575_sd.h"
 #include <linux/seq_file.h>
 
-static char cmd_buf[256];
 
 /* for debug zone */
 unsigned int sd_debug_zone[4] = {
@@ -62,6 +61,7 @@ unsigned int sd_debug_zone[4] = {
 };
 
 #if defined(MT6575_SD_DEBUG)
+static char cmd_buf[256];
 /* for driver profile */
 #define TICKS_ONE_MS  (13000)
 u32 gpt_enable;