IB/ipoib: Use P_Key change event instead of P_Key polling mechanism

The current code use a dedicated polling logic to determine when the P_Key
assigned to the ipoib device is present in HCA port table and act accordingly.

Move to use the code which acts upon getting PKEY_CHANGE event to handle this
task and remove the P_Key polling logic/thread as they add extra complexity
which isn't needed.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Acked-by: Alex Estrin <alex.estrin@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>