net/mlx5e: CT: Return err_ptr from internal functions

Instead of having to deal with converting between int and ERR_PTR for
return values in mlx5_tc_ct_flow_offload(), make the internal helper
functions return a ptr to mlx5_flow_handle instead of passing it as
output param, this will also avoid gcc confusion and false alarms,
thus we remove the redundant ERR_PTR rule initialization.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Suggested-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>