net/mlx5: Bridge, refactor eswitch instance usage

Several functions in bridge.c excessively obtain pointer to parent eswitch
instance by dereferencing br_offloads->esw on every usage and following
patches in this series add even more usages of eswitch. Introduce local
variable 'esw' and use it instead.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>