iwlwifi: refactor pci prob flow

This patch refactores pci prob flow. It moves mac80211 registration
to the end, otherwise there is a race between error path in pci_probe
and mac_start.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>