[ATM]: avoid race conditions related to atm_devs list

Use semaphore to protect atm_devs list, as no one need access to it from
interrupt context.  Avoid race conditions between atm_dev_register(),
atm_dev_lookup() and atm_dev_deregister().  Fix double spin_unlock() bug.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Chas Williams <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>