iwlagn: clean up error path in iwl_pci_probe

This avoids triggering a BUG_ON in pci_disable_msi in the error path.
Furthermore remove the first call to pci_disable_device as it is already
called at out_pci_disable_device.

Both issues were introduced in the patch "iwlagn: fix hw-rfkill while
the interface is down".

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>