net/mlx5: Bridge, extract code to lookup and del/notify entry

Following two patterns in bridge code are used in multiple places where
similar code is duplicated:

- Lookup FDB entry from hashtable by address+vid pair.

- Notify software bridge and then delete existing FDB entry.

In order to improve code quality and prepare for following patch series
that also uses described patterns, extract the codes to dedicated helper
functions.

This commit doesn't change functionality.

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