atm: nicstar: Replace spin_is_locked() with spin_trylock()

ns_poll() used spin_is_locked() + spin_lock() to get achieve the same
thing as a spin_trylock(), so simplify it by using that instead. This is
also a step towards possibly removing spin_is_locked().

Signed-off-by: Lance Roy <ldr709@gmail.com>
Cc: Chas Williams <3chas3@gmail.com>
Cc: <linux-atm-general@lists.sourceforge.net>
Cc: <netdev@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>