igc: Use netdev log helpers in igc_main.c

In igc_main.c we print log messages using both dev_* and netdev_*
helpers, generating inconsistent output. Since this is a network device
driver, we should preferably use netdev_* helpers because they append
the interface name to the message, helping making sense out of the logs.

This patch converts all dev_* calls to netdev_*. There is only two
exceptions:
  1) calls wihtin igc_probe (net_device has not been registered yet)
  2) calls in igc_init_module (module initialization).

It also takes this opportunity to improve some messages.
Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>