USB: g_file_storage: parts of file_storage.c moved to separate file

Moved parts of the file_storage.c file which do not reference fsg_dev
structure to newly created storage_common.c file. dump_msg() and
dump_cdb() have been changed to macros to remove fsg_dev reference.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>