mlxsw: spectrum: Remove unnecessary RCU read-side critical section

Since commit 7d8e8f34

 ("mlxsw: core: Increase scope of RCU read-side
critical section"), all Rx handlers are called from an RCU read-side
critical section.

Remove the unnecessary rcu_read_lock() / rcu_read_unlock().

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>