iwlagn: iwl-trans.c can't dereference iwl_priv any more

This reaches encapsulation for this file. In order to reach this:
 * move priv->valid_context to iwl_shared
 * move the last_rejected initialization to the upper layer
 * define a wrapper iwl_nic_config in the upper layer that calls to
   cfg->lib->ops->nic_config

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>