iwlwifi: consolidate the start_device flow

Now there is only one transport function that launch a specific fw:
trans_ops->start_fw. This one replaces trans_ops->start_device and
trans_ops->kick_nic. The code that actually loads the fw to the
device has been moved to the transport specific code.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>