net/mlx5: Add EQ enable/disable API

Previously, EQ joined the chain notifier on creation.
This forced the caller to be ready to handle events before creating
the EQ through eq_create_generic interface.

To help the caller control when the created EQ will be attached to the
IRQ, add enable/disable API.

Signed-off-by: Yuval Avnery <yuvalav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>