net/mlx5e: Fix warn print in case of TX reporter creation failure

Print warning message in case of TX reporter creation failure, only if the
return value is ERR_PTR type. NULL pointer return indicates that
NET_DEVLINK is not set, and the warning print can be skipped.

Fixes: de8650a8 ("net/mlx5e: Add tx reporter support")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>