mmc: dw_mmc: fix spelling mistake in dev_dbg message

Trivial fix to spelling mistake "desciptor" to "descriptor" in
dev_dbg message.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>