cmd_usb_mass_storage: Use 'USB Mass Storage' in the help text

USB Mass Storage is the standard name, so let's use it here.

Suggested-by: Soeren Moch <smoch@web.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>