ixgbe: Modularize SR-IOV enablement code

In preparation for enable/disable of SR-IOV via the PCI sysfs interface
move some core SR-IOV enablement code that would be common to module
parameter usage or callback from the PCI bus driver to a separate
function so that it can be used by either method.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
CC: Don Dutile <ddutile@redhat.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Sibai Li <Sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>