sfc: Replace spin_is_locked() with lockdep

sfc: Replace spin_is_locked() with lockdep

lockdep_assert_held() is better suited to checking locking requirements,
since it only checks if the current thread holds the lock regardless of
whether someone else does. This is also a step towards possibly removing
spin_is_locked().

Signed-off-by: Lance Roy <ldr709@gmail.com>
Cc: Solarflare linux maintainers <linux-net-drivers@solarflare.com>
Cc: Bert Kenward <bkenward@solarflare.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: <netdev@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Edward Cree <ecree@solarflare.com>