igb: move global_quad_port_a from global into local static define

This change moves global_quad_port_a into igb_probe as a static define
since it doesn't actually need to be global.

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>