mmc: debugfs: add description for module parameter

Make it obvious what this is for.
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210604104459.7574-1-wsa+renesas@sang-engineering.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/debugfs.c

@@ -26,6 +26,7 @@
 static DECLARE_FAULT_ATTR(fail_default_attr);
 static char *fail_request;
 module_param(fail_request, charp, 0);
+MODULE_PARM_DESC(fail_request, "default fault injection attributes");
 
 #endif /* CONFIG_FAIL_MMC_REQUEST */