iwlwifi: trans: clear FW_ERROR status in common code

Clear the FW_ERROR status before the common start_fw transport code.
Remove the transport specific clears.

After these patches the FW_ERROR flag is only set and cleared by common
transport code.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>