net/mlx5: HWS, Fix uninitialized variables in mlx5hws_pat_calc_nop error flow

net/mlx5: HWS, Fix uninitialized variables in mlx5hws_pat_calc_nop error flow

In mlx5hws_pat_calc_nop(), src_field and dst_field are passed to
hws_action_modify_get_target_fields() which should set their values.
However, if an invalid action type is encountered, these variables
remain uninitialized and are later used to update prev_src_field
and prev_dst_field.

Initialize both variables to INVALID_FIELD to ensure they have
defined values in all code paths.

Fixes: 01e035fd0380 ("net/mlx5: HWS, handle modify header actions dependency")
Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Link: https://patch.msgid.link/20250825143435.598584-4-mbloch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>