ixgbe: zero out mailbox buffer on init

This patch initializes the msgbuf array to 0 in order to avoid using random
numbers from the memory as MAC address for the VF.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>