iwlwifi: return error if loading uCode failed

In "iwlwifi: consolidate the start_device flow"
the code flow changed and the firmware is now
loaded by the transport layer, but the change
unfortunately lost error checking -- restore.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>