igb: Fix panic with NICs with 1000BASE-SX PHY

This patch eliminates a kernel panic with the igb driver in 2.6.25-rc2 when
running on a Intel 82575 Ethernet controller with a 1000BASE-SX PHY.  The
panic does not happen with the 1000BASE-T PHY, only with a SX connection.

Signed-off-by: Bill Hayes <bill.hayes@hp.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>