igb: initialize mailbox function pointers prior to phy init

The igb driver is currently initializing the mailbox function pointers
after the phy.  This causes issues as the phy init will return from the
function early if there is no phy present.

To resolve this I have moved the function pointer init to a location prior
to the phy initialization so that serdes based adapters can also make use
of SR-IOV.

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>