iwlwifi: fix error flow in iwl*_pci_probe

Both the agn and 3945 drivers has some problems with dealing with
errors in their probe functions. Ensure that a goto will undo only
things that was done before the goto was called.

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