igb: warn if max_vfs limit is exceeded

Currently there's no warning printed when max_vfs > 7 is specified with
igb and the maximum of 7 is silently enforced. This patch prints a
warning and informs the user of the actions taken.

Signed-off-by: Stefan Assmann <sassmann@redhat.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>