USB: storage: Use USB_ prefix instead of US_ prefix

This commit changes prefix for some of the USB mass storage
class related macros (ie. USB_SC_ for subclass and USB_PR_
for class).

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>