igb: do not check for vf_data if we didn't enable vfs

The driver is currently dumping a message in the log about failing to
allocate vf data when max_vfs is equal to 0.  This change makes it so the
error message is only displayed if we set max_vfs to a non zero value.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>