iwlwifi: trans: don't call the trans-specific ref/unref directly

It's cleaner to always call the iwl_trans_ref/unref() functions
instead of sometimes calling the trans-specific ops directly.  This
also prepares for moving some of the code from the trans-specific ops
to the common trans code.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>