mmc: mmc_test: mention that '0' runs all tests

I had to use the source to determine what I need to write to 'test' so
that all tests are run. Let's mention this explicitly in 'testlist'.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/card/mmc_test.c

@@ -2829,6 +2829,7 @@ static int mtf_testlist_show(struct seq_file *sf, void *data)
 
 	mutex_lock(&mmc_test_lock);
 
+	seq_printf(sf, "0:\tRun all tests\n");
 	for (i = 0; i < ARRAY_SIZE(mmc_test_cases); i++)
 		seq_printf(sf, "%d:\t%s\n", i+1, mmc_test_cases[i].name);