mlxsw: Narrow the critical section of devl_lock during ports creation/removal

No need to hold the lock for alloc and freecpu. So narrow the critical
section. Follow-up patch is going to benefit from this by adding more
code to the functions which will be out of the critical as well.

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