i40evf: properly handle multiple AQ messages

When we receive an admin queue message, the msg_size field in the event
struct gets overwritten. Because of this, we need to reinit the field
each time we go through the loop. Without this we may receive truncated
messages due to the firmware thinking we have insufficient buffer size.

Change-ID: I21dcca5114d91365d731169965ce3ffec0e4a190
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Patrick Lu <patrick.lu@intel.com>
Tested-by: Jim Young <jamesx.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>