usb: gadget: storage: check for valid USB_BULK_GET_MAX_LUN_REQUEST

The latest USB-IF CV tester checks for a valid length for this
request.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>