USB: storage: convert to use module_usb_driver()

Now that Joe cleaned up the init/exit functions, we can just get rid of
them entirely and use the proper macro that almost all other USB drivers
now use.

Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>