USB: uas: add locking

Add spinlock to protect uas data structures.

[ v2: s/GFP_NOIO/GFP_ATOMIC/, better don't sleep when holding a spinlock ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>