USB: Stall control endpoint when file storage class request wValue != 0

This patch makes the File Storage Gadget stall the control endpoint
when a MSC class request is made with wValue != 0.  This change makes
some MSC compliance test warnings disappear.

Signed-off-by: Luis Lloret <luislloret@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>