i40e/base: fix uncertain event descriptor issue

In one obscure corner case, it was possible to clear the NVM update
wait flag when no update_done message was actually received. This
patch cleans the event descriptor before use, and moves the opcode
check to where it won't get done if there was no event to clean.

Fixes: 8db9e2a1b232 ("i40e: base driver")

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>