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_dataflash_mmc_mux.c b/common/cmd_dataflash_mmc_mux.c
index fc23ed6..1678d6e 100644
--- a/common/cmd_dataflash_mmc_mux.c
+++ b/common/cmd_dataflash_mmc_mux.c
@@ -58,7 +58,7 @@
 
 U_BOOT_CMD(
 	dataflash_mmc_mux, 2, 1, do_dataflash_mmc_mux,
-	"dataflash_mmc_mux\t- enable or disable MMC or SPI\n",
+	"enable or disable MMC or SPI\n",
 	"[mmc, spi]\n"
 	"    - enable or disable MMC or SPI"
 );