If debug() is not used, then the whole content of debug(...) will
be removed by the preprocessor, which will result in the following
warning. This patch adds __maybe_unused annotation to fix this.

drivers/mmc/mmc.c: In function ‘mmc_init’:
drivers/mmc/mmc.c:1685:11: warning: variable ‘start’ set but not used [-Wunused-but-set-variable]
  unsigned start;

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>