IB/mlx5: Restrict 'DELAY_DROP_TIMEOUT' subtype to Ethernet interfaces

Subtype 'DELAY_DROP_TIMEOUT' (under 'GENERAL' event) is restricted to
Ethernet interfaces. This patch doesn't change functionality or breaks
current flow. In the downstream patch, non Ethernet (like IB) interfaces
will receive 'GENERAL' event.

Fixes: 5d3c537f

 ("net/mlx5: Handle event of power detection in the PCIE slot")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>