net/mlx5e: Check ets capability before ets query FW command

On dcbnl callback getpgtccfgtx, the driver should check the ets
capability before ets query command is sent to firmware.
It is valid to return from this void function without changing in/out
parameters, as these parameters are initialized to
DCB_ATTR_VALUE_UNDEFINED.

Fixes: 3a6a931d ("net/mlx5e: Support DCBX CEE API")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>