net/mlx5: Stay in polling mode when command EQ destroy fails

During unload, on mlx5_stop_eqs we move command interface from events
mode to polling mode, but if command interface EQ destroy fail we move
back to events mode.
That's wrong since even if we fail to destroy command interface EQ, we
do release its irq, so no interrupts will be received.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>