iwlagn: fix rmmod crash

priv->bus.bus_specific pointer is used after priv structures was freed,
in iwl_pci_remove(), what make ugly rmmod crash. This bug was introduced
by current pci changes.

On the way remove fake check, if prober error code is returned from
.probe() function, .remove() will never be called be null drvdata.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>