usb-storage: export symbols in USB_STORAGE namespace

Modules using these symbols are required to explicitly import the
namespace. This patch was generated with the following steps and serves
as a reference to use the symbol namespace feature:

 1) Define DEFAULT_SYMBOL_NAMESPACE in the corresponding Makefile
 2) make  (see warnings during modpost about missing imports)
 3) make nsdeps

Instead of a DEFAULT_SYMBOL_NAMESPACE definition, the EXPORT_SYMBOL_NS
variants can be used to explicitly specify the namespace. The advantage
of the method used here is that newly added symbols are automatically
exported and existing ones are exported without touching their
respective EXPORT_SYMBOL macro expansion.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>