Move cmd_buf for dbg under ifdef to fix compilation warning:
drivers/mmc/host/mtk-mmc/dbg.c:51:13: error: 'cmd_buf' defined but not used [-Werror=unused-variable]
51 | static char cmd_buf[256];
| ^~~~~~~
cc1: all warnings being treated as errors
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>