USB: cdc-wdm: use two mutexes to allow simultaneous read and write

using a separate read and write mutex for locking is sufficient to make the
driver accept simultaneous read and write. This improves useability a lot.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Cc: stable <stable@vger.kernel.org>
Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>