USB: storage: refactor endpoint retrieval

Use the new endpoint helpers to lookup the required bulk-in and bulk-out
endpoints and the (typically) optional interrupt-in endpoint.

Cc: usb-storage@lists.one-eyed-alien.net
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>