net/mlx5e: Fix AF_XDP TX timestamp teardown NULL dereference

During XSK TX queue teardown, outstanding descriptors are completed
without a CQE. If one requested a TX timestamp, the completion path
passes that NULL CQE to mlx5e_xsk_fill_timestamp(), which dereferences
it.

Return zero when no CQE is available, indicating that teardown did not
produce a TX timestamp.

Fixes: ec706a860eba ("net/mlx5e: Implement AF_XDP TX timestamp and checksum offload")
Signed-off-by: Prathamesh Deshpande <prathameshdeshpande7@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260926165402.5902-1-prathameshdeshpande7@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>