various cmd_* files: remove the command name from the help message

removed the command name from the help message as it is already printed.
for cmd_mmc also rewrote the message a little bit

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
diff --git a/common/cmd_mgdisk.c b/common/cmd_mgdisk.c
index 51c994b..d99af2d 100644
--- a/common/cmd_mgdisk.c
+++ b/common/cmd_mgdisk.c
@@ -61,7 +61,7 @@
 
 U_BOOT_CMD(
 	mgd,	5,	0,	do_mg_disk_cmd,
-	"mgd     - mgine m[g]flash command\n",
+	"mgine m[g]flash command\n",
 	": mgine mflash IO mode (disk) command\n"
 	"    - initialize : mgd init\n"
 	"    - random read : mgd read [from] [to] [size]\n"