net: cassini: use nested lock annotation

In the cas_lock_tx function we acquire multiple locks in a loop and
need to use nested lock annotation to prevent lockdep warnings.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>