USB: Move USB Storage definitions to their own header file

The libusual header file is hard to use from code that isn't part
of libusual.  As the comment suggests, these definitions are moved to
their own header file, paralleling other USB classes.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
[mina86@mina86.com: updated to use USB_ prefix and added #include guard]
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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