USB: gadget: file_storage: reuse definitions from a header file

This commit changes storage_common.c and file_storage.c to
reuse definitions from linux/usb/storage.h header file.

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>