ixgbe: Return error on failure to allocate mac_table

Add a check to make certain mac_table was actually allocated and is not
NULL.  If it is NULL return -ENOMEM and allow the probe routine to fail
rather then causing a NULL pointer dereference further down the line.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>