mmc: Fix warning if debug() is not used

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>