net/mlx5: E-Switch, Change mode lock from mutex to rw semaphore

E-Switch mode change routine will take the write lock to prevent any
consumer to access the E-Switch resources while E-Switch is going
through a mode change.

In the next patch
E-Switch consumers (e.g vport representors) will take read_lock prior to
accessing E-Switch resources to prevent E-Switch mode changing in the
middle of the operation.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>