net/mlx5: qos: Rename vport 'tsar' into 'sched_elem'.

Vports do not use TSARs (Transmit Scheduling ARbiters), which are used
for grouping multiple entities together. Use the correct name in
variables and functions for clarity.
Also move the scheduling context to a local variable in the
esw_qos_sched_elem_config function instead of an empty parameter that
needs to be provided by all callers.
There is no functional change here.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>