iwlwifi: add fw_alive to transport layer API, kill tx_start

Define a new handler in the transport layer API: fw_alive.
Move iwl_reset_ict to this new handler, and move the content
of tx_start to this handler.

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