net/mlx5e: Replace async events spinlock with synchronize_irq()

We only need to flush the irq handler to make sure it does not
queue a work into the global work queue after we start to flush it.
So using synchronize_irq() is more appropriate than a spin lock.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>