ixgbe: wrap netdev_priv() usage

Wrap use of netdev_priv() in order to change the allocator of the device
private structure from alloc_etherdev_mq() to the devlink in next commit.

All but one netdev_priv() calls in the whole driver are replaced, the
remaining one is called on MACVLAN (so not ixgbe) device.

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Bharath R <bharath.r@intel.com>
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>