usb: gadget: storage: check for valid USB_BULK_RESET_REQUEST wLength

The USB-IF CV compliance tester is getting stricter, and it would
be valid for it to fail a mass-storage device that accepts an
invalid USB_BULK_RESET_REQUEST request. Although it doesn't do
that yet, let's be proactive and fix that now.

Suggested by Alan Stern.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>