iwlcore: register locks

Add new lock to be used when accessing some registers. Also move
    the register lock and iwl_grab_nic_access inside the function for register access. This
    will prevent from forgetting to hold locks and nic access in the right way and make code
    easier to maintain.

    We over use the priv->lock spin lock and I guess we need to add new
    one for Tx queue after that we might need to change most of these lock to
    BH and just keep priv->lock as irq type.

Signed-off-by: Mohamed Abbas <mohamed.abbas@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>